So your right to as d+15/32=b so b= 4 15/32"
As long as I’ve been doing engines, manufacturers always use a known always constant measuring point. That in this case being the gasket surface to the outside fan hub mount. It’s the only measurement that makes sense. That would always put the outer hub surface in the same place after a rebuild. Sure enough I went out with my calipers and a flat surface ,bam gasket surface to outside surface of fan mounting hub all 3 = 4” 15/32
give or take a 32nd