Because of the way LiPo's work, you really cannot go by the voltage levels.

In effect LiPo voltage decreases initially under load, then levels out for quite a long time, before the voltage curve drops off abruptly as they approach lower capacity.

Instead of using voltage, charge the pack up to full capacity, then discharge the LiPo pack by about 60% of full capacity.

So if you have a 2200mAh LiPo pack, charge it up FULLY, so you see 2200mAh or so being put back in, then discharge the pack removing about 900mAh of current.

LiPo's have a fairly slow self discharge rate, typically LESS than 5-8% per MONTH(!), so doing this gives you about 7 months of storage. You'll need to recharge the packs after 7 months or so.

You never want to let the LiPo capacity drop under 10% which is indicated by a corresponding and abrupt voltage drop under load.

Drop the voltage and capacity too much and you've ruined the LiPo packs.