Casting Armor

You touch a willing creature's clothes or armor, and a protective magical force surrounds it until the spell ends. The target's clothe's or armor's AC becomes 13 + the casters spell modifier. The spell ends if you dismiss the spell as an action. 

Casting this spell when you are level 10 makes the armor or clothing's AC = 13 + The casters Spell Modifier + Half Proficiency of the Caster
Casting this spell when you are level 15 makes the armor or clothing's AC = 13 + The casters Spell Modifier + Proficiency of the Caster