Your thinking looks sound to me, that seems like that's too much drop for the draw you're presenting.

I will comment that you can get a really nice LED flashlight with two 18650 cells and a charger off eBay for < $20. I have one on my helmet now. It's completely waterproof and pocketable and it puts out a LOT of light, probably on the order of 300 lumens. Given that, I don't think it's really worth screwing around with DIY solutions anymore. I did the whole overvolted MR16 light hooked to a lead-acid cell, DIY HID Trailtech lights, etc, but those days are over.