BBL,
On one CR123A cell it highly depends on the forward voltage of the Luxeon you have in the KL4. Do a search on "TW4" and you will get lots of info on the KL4 running on one CR123A. If you want the full 65 lumens of output, you need to use a rechargeable R123 Li-Ion cell that will provide a nominal voltage of 3.7 volts.
Jan