I changed the type of the celestial heal to SpellType_Buff and gave it a low priority and it worked the way I wanted with the celestial heal being cast and then the regular heal if there was a need for a booster, but since the npc was casting the celestial heal every time it wore off, it very quickly ran out of mana  :( 
The proc rate was above 0 but it didn't seem to matter. 
		
	
		
		
		
		
		
		
		
		
		
	
		
		
	
	
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