Well, evasion either prevents damage or reverses it and the helm of protection does the other, if both activate on Eye Ra he gets an hp boost.
Example: Ak hs = ~134 for helm of protection you need 150 hp+
if eye ra starts with 200 and gets hit in the head his hp is now 324 if both activate. 33% chance. If helm activates hp remains at 200. It's easier to see when someone unloads a tmp clip into your head, mote chances to activate.