Reduce Projectile Damage

I'm trying to work on a procedure that will allow me to reduce the damage of incoming projectiles, similar to the "Projectile Protection". I'm attempting to add an Armor Set bonus for a particular armor set, that will reduce said damage while still allowing players to put enchantments on the armor.

I have half the procedure, and am able to check for the armor - but the problem I'm having is how to write the procedure to:

1. Detect the incoming damage as a projectile

2. Reduce the incoming damage IF it is a projectile.