the math here would net you 7" BUT instead of just assuming you need to measure from the keel to the top of the transom and from the anti vent plate on the motor to the bottom of the transom hooks....... 
Understand that you will set only your lowest possible height here so build to put the anti vent plate 1" below the expected desired height... in this case since you expect to need it 2" above the keel you would aim for a minimum of 1" above the keel.... Then you can raise the motor one hole at a time till you get the best performance. 
 
 
example: if your motor shaft is actually 26" and your transom is actually 19 then you would need 9" -1" for a total of 8"
		 
		
		
		
		
		
		
		
		
	
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