Wide rear rallyes

[OneBad71]

Hello, I am starting over on a 71C10 build I started 10 years ago. Trying to figure out rear rim backspace because they need to be special ordered. I have the stock 12 bolt rear and I have added the wide Tubs. My plan is to go with 12inch wide US Wheel Rallye's in the back. They will do a custom backspace for me. I want to run the Hoosier Pro street 29x15x15 tires. I have been trying to figure it out but don't want to make a mistake on the rim backspace. So far i'm coming up with a 8.5in back space ? Has any one done this same tire and wheel combo on the wider rear axle like mine ? Based on my homemade Jig going off the drum face, I would have 8.5in in and 4.5in out. (13"overall rim width) Any help would be appreciated. Thank you