Battery health

See the 'Your battery's maximum capacity' section in:

iPhone Battery and Performance - Apple Support

"A normal battery is designed to retain up to 80% of its original capacity at 500 complete charge cycles when operating under normal conditions. The one-year warranty includes service coverage for a defective battery. If it is out of warranty, Apple offers battery service for a charge. Learn more about charge cycles."

The rate at which your battery health decreases is quite dependent on usage and charging. So it's hard to say what 'normal' really is. The batteries are designed to make it to 500 charge cycles and still have 80% of their capacity.

So assuming you use ~80% of your charge each day, 88% could be perfectly normal.