David,  
I measured it a second time.  The way I'm measuring it isn't super precise, but it's reasonably close.  First, a picture:
  
From the bottom of the "B" in Boxster to the edge of the trunk lid is ~55.5mm.  From the bottom of the "S" (at its lowest point) to the trunk lid is ~50.8mm. 
 
Hope this helps!
		  
		
		
		
		
		
		
			
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				2000 Boxster S,  6 speed, Sport Package, Litronics, LED tail lights, LNE IMS-B, OBC, Skybreaker wind deflector, Arctic Silver/Graphite Grey
			 
		
		
		
		
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