2 things:
1. Use a current shunt, like 10mOhms of 16AWG wire to know the specific voltage drop to calculate the current, on the tailcap. That’s my method, as I don’t need a current meter for now.
2. Use a hall effect clamp meter:
Yeah, but if you get 2000 lumens from four emitters, that’s only 500 lumens per emitter average. Even at 2600 lumens, it would only be 650 lumens per emitter. If it is using 18A to get there, that’s 4.5A per emitter, for an efficiency of less than 50 lumens per watt! That’s atrocious!
According to CREE PCT, the old XM-L in the LOWEST BIN, got an efficiency of 50.7 lumens per watt at 3 amps. So, this 219C is getting less than 50 lumens per watt at 4.5 amps, marginally better than the lowest bin XM-L of decades ago. 
Jason, our measurements do match up. About 8 amps top of ramp, and 18-20A (on startup).
I don’t have a way of measuring lumens though…
weights without battery:
Astrolux S43
Astrolux S43S copperhead
Well, I mean, the 219C has an efficiency of 69,6lm/W at 4,5A.
If you count the inefficiencies of the circuit, like the dual phosphor bronze springs having higher resistance than a single BeCu spring for some reason, and the less efficient optics, and thinner traces of the driver, no wonder the S43S is less efficient than even the Emisar D4.
Don’t forget the 219Cs in Astrolux’s tests may be lower binned than Intl Outdoors’ LEDs, and what not.
Just got my S43S NW today. My Texas_Ace lumen tube measured the same 1200 top of ramp, 2000 turbo using keeppower 18350 cells. Drains the cell really, really quick 
I bought the copy cat light with scalloped tube.
Yes it gets quite hot rather quickly as can be seen here…
I’d recommend shaking your new lights and listening for a slight rattle in the head.
Mateminco is not a copy cat. It’s the same light from the same company. Like Honda and Acura, etc…
I recommend everyone with this light to reset the temperature stepdown. In stock form it gets too hot then steps down to about 60 - 90 lumen. That’s no good.
I reset mine to step down a bit sooner. Now it acts much nicer and takes multiple small step downs over time.
I did a video. The 30Q was not fully charged due to a few turbo runs the day before so turbo output is a bit low.
35 seconds at Turbo
23 seconds at 550lm
23 seconds at 285lm
23 seconds at 130lm
Then steady at 87lm
I’m not sure I understand correctly, do you mean if the thermal temperature threshold is set lower, the stepdown will be in smaller steps? Though would step down faster when in Turbo mode?
Yes, that’s correct.
I had to add some music to cover a background conversation. Sorry. There’s no dialog, so you can turn the sound off if you want.
Video on how to change thermal step down.
I've been puzzled by these references in some reviews to the sudden fierce thermal regulation step down from turbo of the S43 and S43S. My S43S as out of the box unaltered (ordered early November, arrived about a week ago) has always stepped down in a sequence of steps like those you cite after lowering the thermal regulation temperature. The first step down from turbo varies quite a bit depending on the state of charge of the battery and which battery. I've seen a 43 sec minimum and a 110 sec maximum. After that the subsequent steps down always take place at regular intervals of around 23 seconds. It's the same behaviour whether I use a 30Q 18650 or an 18350, except that a fully charged 18350 can't reach the same initial turbo brightness or short time as the 30Q.
I’m sure Tom E could explain the NarsilM thermal stepdown behavior in detail.
Keep in mind that the thermal sensor for the driver is built into a chip on the driver itself, the Atmel ATtiny MCU, so there is a certain delay and inaccuracy associated with it. Still, it works pretty well on lights that heat up a bit slower. On these small lights that heat up super fast it can sometimes cause the software to act funny.
ToyKeeper has tweaked the Anduril UI, which is going to be used on the forthcoming FW3A, to be more of a thermal control as opposed to a thermal stepdown. The FW3A also gets hot very quick, but once it cools it can actually go back up in brightness. It still uses the ATtiny chip which is what most of the BLF user interfaces are based on.
I have some notes on NarsilM v1.0 and it says the mcu samples the temperature after the initial stepdown every 45 seconds in order to give the temperature a bit of time to stabilize. It seems newer versions have cut this time in half. Probably every 22.5 seconds, which is why we see it adjust in 23 second increments.
Hhhmm, guess I should know, but feels like forever ago - I should revisit the code, refresh my ol memory chips - they've been losing bits every day... 
That’s my experience, too. My only complaints are that
1. It steps down too far. As mentioned, you end up with less than 100 lumens by the time its done.
2. It never steps back up after it cools off.
Okay, I get that (2) is probably not going to be implemented in a budget light, so I’m okay with that. But (1) should really not happen. Perhaps the firmware should have a “pause” limit of something like 300 lumens, and stay there for a few minutes before it tries further step-downs.
OTOH, last night I was out walking in –16C temperatures, and used turbo for several minutes without any step-downs at all. So, the temperature sensor works okay. Unfortunately, turbo produces way too much heat for any decent run-times at room temperature. But outside in winter, it’s much more useful.
If it’s doing one giant stepdown, try what I mentioned earlier. Reset the stepdown temperature a little lower. After doing this I got several smaller stepdowns instead of one big one…
I believe the head heats up quicker than the thermal sensor heats up. So by the time the thermal sensor gets to 55°C (IIRC), the head is already well beyond this. Resetting the sensor based on how hot the head feels seems to work better than the factory setting.
problem is the MCU firmware can't detect lumens output - all it can control is relative output and it doesn't know even what amps is going out, yet alone lumens, and the firmware, least how I wrote it, is for generic lights, not specific LED, host, and battery configurations.
Also the temp in the MCU is disconnected from the temp at the LED(s) and temp you are feeling on the body. It's a very imperfect system. External temp sensors at the proper places would help for sure, but still the algorithms aren't trivial.
However Zebralight does their thermal control, works extremely well. I’m guessing they use some integrated approach (and maybe expensive) that isn’t practical in this light.
As a possible partial-solution, could the firmware you wrote not be tweaked for whatever host is using it? For example, in this light, it could start at a lower temperature, and each time it steps down it could raise the temperature threshold a bit. That might slow down the step-downs, and allow the light to settle down at a higher output.
Zebralight designs their firmware for their individual flashlight models - we/I don't, plus they have regulated output so temps are predictable,can be modeled - ours are not, we use FET designs in extreme ranges of power/heat and various small/large hosts. I don't know how you can compare - it's totally different. I would love to have just one platform with regulated output and decent temp sensors to write firmware for - it would be so much easier.
The Attiny85 built-in temp sensor is just not reliable. Here's a table of values I've used for recent offset calibrations in NarsilM for the ATtiny85:
// Temperature Calibration Offset - value is added to reading, higher the #, higher the reading:
#define TEMP_CAL_OFFSET (4)
// 4 about right for the C20C#2 (triple)
// -11 about right for the C20C, kludgy driver (think MtnE modded)
// -18 adjusted for the X7R DEL 17mm driver, piggybacked
// -12 rough guess for the C8F 21700 triple
// -12 rough guess for the X6R triple
// -14 about for the TA driver for the M8
// -3 try for SP03
// 1 about right for the C8F #1
// -12 guess for the JM70 #2
// -19 is about right for the Lumintop SD Mini, IOS proto driver
// -3 Decided to use this for Q8 production
// -6 BLF Q8 Round 3 - blinks 29C w/3 setting for 20C (68F) room temp
// -2 try for the Manker U21 (LJ)
// -2 works for the Warsun X60 (robo) using the 17 mm DEL driver
// -1 try this for proto #1, OSHPark BLF Q8 driver
// 3 about right for BLF Q8 proto #2 and #3, reads ~20 for ~68F (18C)
// -12 this is about right on the DEL DDm-L4 board in the UranusFire C818 light
// -11 On the TA22 board in SupFire M2-Z, it's bout 11-12C too high,reads 35C at room temp, 23C=73.4F
// -8 For the Manker U11 - at -11, reads 18C at 71F room temp (22C)
// -2 For the Lumintop SD26 - at -2, reading a solid 19C-20C (66.2F-68F for 67F room temp)
So the range from above is from -19 to 4, a 23C swing just in offset - who knows how badly calibrated it is in gain, then how sensitive it is to change, etc. It's really an awful sensor to use for anything much. Dr Jones spent weeks/months on developing a PID based algorithm for thermal regulation and I still hear there's problems with it.
so its a hotrod like the d4 that pulls like 20A on turbo, no big suprise here jasonww 
I kinda expected it to get hot very fast on highest.