Your math is fine. Spell AC is weird. If you take off an item worth 40+ AC the change is huge, but with a spell it's much smaller. 
 
If you look at #mystats, under mitigation AC, there is a field for "spell." 
 
On my level 60 GM char using crappy GM armor, my mitigation AC is 360. If I take off my 40 AC breastplate it drops to 306. When I cast Resolution it goes up to 373 (up from 360). However, "spell" goes from 0 to 55 (reflecting the full value of Resolution). 
 
Someone better with the actual AC formula might can chime in, though. 
		
	
		
		
		
		
		
		
			
		
		
		
		
		
		
	
		
		
	
	
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