Battery drain

Doesn't matter. That amount of battery "drain" is not unusual. All batteries lose maximum capacity over time ... even if not used heavily. If you disagree, please make a Genius Bar appointment at your local Apple Store or Apple Authorized Service Provider and have them take a look. Chances are they will tell you the same thing as I have already.

This would not be out of the ordinary. Apple doesn't consider an iPhone's battery requires replacement unless it cannot maintain, at least, an 80% charge after 500 full recharging cycles. Yours is not even close.

Ref: iPhone Battery and Performance - Apple Support