As I said above, I have a PVC light box - guess you call it a tube? One of the original ones, before TA was around, before Dale got one. Because a rt. angle light doesn't fit, I took a somewhat similar light with about the same optic width, the SC31 Pro, and used it to calibrate the position and configuration. I held the light centered over the glass, but up a couple inches above the glass - normally the flashlight sits flush on the glass with dark gray foam blocking the rest of the opening. The conversion factor I came up with was adding 5.8%. It's approximate for sure, but if you look at XPL HD LED tests, at 6.6 amps, by TA here, and djozz here, 1500 lumens is well under what they tested.
It could be my driver has an issue in that something that should be throttling back to the FET on turbo is not working, like a resistor that is short circuited.
I haven't seen any amp readings posted on the HD20, so that's the ultimate data to compare. Btw, it's not easy to do an amp reading unless you got a third hand helping. I was able to do it myself in my lap, and able to apply enough pressure to get the spring about fully compressed, maybe as much as when the tailcap is tightened down.