Should I cycle my Li-ion from 100% to 80% or from 60% to 40% to maximise life?
Both extremes of full charge, and full discharge, reduce the life of a lithium cell. Operating the cell between nominally 80% and 30% SOC will give a greatly improved lifetime.

NASA apparently charge their Lion's to 3.92v instead of the rated 4.1v to maximise their life.