Never believe the lumen claims you see on websites selling lights…
Usually they quote the theoretical max for the emitter if driven at the max ‘safe’ current – but your driver won’t deliver that current, the cells probably won’t deliver that current, and without more thermal management everything would probably just fry anyway.
For a rough estimate you can check flashlightwiki.com, but you will still need a multimeter to see what amps your driver is drawing at each mode… and this I think will change as the voltage drops, especially on HI mode.
Anyway, I’m impressed with the XM-L2… much brighter than my XML-T6s, but some of that is down to higher current, especially in MID mode. For £8 it’s a real bargain.