BTW a cutoff at 11v would be fine.
Here is my test of voltage vs time. The battery was a 14.4v 4500 mAH NiMH, and the load was a 13W TrailTech HID and a small amber strobe (about 250 mA drain I think).

http://www.hauntedfrog.com/HID%20light%20voltage.pdf

As you can see, with a 14.4v battery, once you hit 13v, you're heading south fast. If you multiply by 10/12 (for 10 batteries instead of 12), your drop point will be around 10.8v. Really you're about to start tanking by that time and have very little life left.