Well typically with any battery source, if you do a charge and discharge of that device you will get more use out of it. So allow it to drain to a fail 3-4 times then go ahead and charge it whenever. Just trying to get a full charge out of the battery.
I'm not sure on the amount your not supposed to go below, but charge to 100% exactly. Don't leave it plugged in all night, because after time it starts to drain the battery apple uses. A complete 0-100% should take about an hour