Considering the time, effort, and expense associated with the planes we fly, I'm curious about what "rules of thumb" people follow for determining when it's time to replace battery packs. I check my receiver and transmitter packs before every flight and cycle them all a couple times a year. Several of my packs are coming in at 90-95% of capacity. At what point should they be replaced? 85% of capacity, 80%, ....? What guidelines do you follow?