The easiest way is to measure amps at the emitter. I (almost) always do that on my mods when putting them back together.
As for the stock light.
K40M stock emitter amps
4,86 startup
4,77 10 sec
4,71 30 sec
4,66 1 min (Before stepdown)
4,18 1 min 10 sec (After STEPDOWN)
4,16 2 min
4,14 2 min 30 sec
4,12 3 min 30 sec
4,11 4 min 30 sec
4,10 5 min
So what some might think is heat sag, is a combination of driver giving less amps to the emitter over time + stepdown + natural heat sag.
Ive mentioned how lumen drops in my comparison thread with other lights. Pictures and beam shots there too in case people have not seen.
"K40M drops about 210 lumen at 1 minute 10 seconds. From 1 minute 10 seconds and to 5 minutes it drops another 80 lumen.
....
I put some protected NCR18650BD at 3,6V in both lights. K40M showed 2856 lumen (basically the same as with fresh batteries)"
Ill probably post mod results and more data in your mod thread once my light is done and properly tested. My light is currently in pieces. Not sure what day Ill put it together. (yes, the small sealed screws at the driver compartment were a PITA. Supbeam X40 did not have sealed screws.)