Can't help ya there. The response I've gotten to similar questions around here are basically "hook it up and see if it works."
LEDs get tricky as their forward voltage varies with current more than incan bulbs--their power consumption is not constant.
Maybe one of the guys with XM-L2 lights can lend some experience on what voltage drop their LEDs are experiencing at a certain current level; then, you can scale up to your planned ~26V (which sounds a bit high for 6x Lithium cells....)
