I decided to stop at 28 - 29 ml. The mod had been showing 20% for a while, though hadn't yet generated a low battery warning.
The xtar charger shows both batteries were at 3.5v, and they're charging at the same rate, consistent with a balanced discharge.
Tomorrow I'll start running the second pair of 20700b, see how long they last.
20700 is a new format, just a few mods out that can use them. Sanyo makes the 20700A (high Amps, moderate mAh) and 20700b (only15 - 16A, 4000mAh). Ijoy also has a good 20700 (not sure whose cell it is). I got the 20700b from battery junction.