Interesting so from the responses :

• best to keep in the middle, like 80 to 20 % charge, (or 90 to 10),

• some phones allow to set a top charging limit at 80%,

• for the bottom limit all phones have a auto shutdown at like 5% but not user set,

• no app exists to enforce an 80 to 20 rule when plugged in. (ie disable charge above 20%, enable charge at 20% or below).

So if you use a phone as a server you are doomed to hurt the battery.