At what point is it best to recharge the iPad battery

Here's a pretty well known reference...

Similar to a mechanical device that wears out faster with heavy use, so also does the depth of discharge (DoD) determine the cycle count. The shorter the discharge (low DoD), the longer the battery will last. If at all possible, avoid full discharges and charge the battery more often between uses. Partial discharge on Li-ion is fine; there is no memory and the battery does not need periodic full discharge cycles to prolong life, other than to calibrate the fuel gauge on a smart battery once in a while. Read more about Battery Calibration.

Interesting.

That writeup makes it clear that Apple, Bose, Black & Decker, Nokia and Samsung personnel do not understand the characteristics of lithium batteries!