I have a voltwatch that I check each plane with before the day begins, if I haven't charged them since the last outing. Just by quick calculation, I know about what (guesstimate) percentage of battery is used each flight for every plane. Based on the capacity of the battery, I can figure how many flights I can have before I should worry, then subtract one.

All of this seems inperfect, but I pretty much know that I can get 4-5 flights on every plane I have safely, but I fly no more than 4.

I usually can't go flying for much longer than about 3-4 flights anyways, including a lot of just sitting around enjoying being at the field. If I have more time, I take more than one plane making sure I don't fly either one excessively.

There's my crude mental Voltmeter for ya!