Richmond 3.90 gears pinion depth GM 8.5 10 bolt rearend

I was bewildered by the pinion numbers on the pinion gear for the set of gm 10 bolt 3.90 gears I picked up from Summit. I spoke to Richmond and they gave me the information needed to calculate the pinion depth given the numbers written on the gear were a total measurement from center including the gear head. I typed in wrong pinion numbers and other search types but did not get the answer I needed. I hope this helps.