iPhone 6 charging best practices
Yes, the ideal time to charge your phone is around 50% depletion. Ars Technica explains why:


  Full discharges put a lot of strain on the battery, and it's much better practice to do shallow discharges to no lower than 20 percent. In a way, this is like people running for exercise— running a few miles a day is fine, but running a marathon every day is generally not sustainable.


Turning off the device may assist slightly in lowering heat, which in turn may extend the life of the battery by a minuscule amount, but it is not really enough to worry about.