[DeTomaso] Wheel Order

[Will Demelo]

I'm about to F%^$ blow a gasket.Can someone help me calculate my backspacing for the VERY expensive wheels I just ordered? Talk to 3 different vendors and get 5 different answers.
I ordered my wheels from Bob Byers just an hour ago. I have glass GR4 flares that were made by him and purchased at Hall.
I was told by Larry Stock that the GR4 flares require the same backspacing as GT5 cars which is 4,625" frt (based on 10" rim) and 4.75" rear (based on 13" rim).
I just ordered 12.5" rears and 9.5" fronts. Bob said the rear backspacing is ~6". The fronts were~5". I didn't record it because they've done dozens of GR4 conversions.
I just did my calculations and came up with 3.375" rear and 3.25" frt. BIG difference. 
WTF?
Can I send someone my hub/flare dimensions to calculate? Maybe (probably) I'm missing something????
Contact me offline for the sketch if you can help.
Will