I saw the results of an experiment where they tested the 80% rule, and it literally doubled the charge cycles they got from 800 for 100% every time to 1600 for 80 percent. As I understand it, it is best to charge fully every now and then to let the BMS balance the cells etc, but it's better for the battery not to be charged to 100% most of the time, and you definitely don't want to leave it at 100 for any length of time.