rim backspacing ?s

[NOmulletLs7]

ok so i have a strange 12 bolt rearend i am trying to put it in my 98 trans am and im wondering about what backspacing i will need cause the rearend is narrow 2 inchs. i want to run some pro stars, like a 15x10 and ive read on a few threads on her that with a stock axle u should run like a 7.5 in backspacing so im thinkin i would just need 1 in less then that but im not possitive, if anyone has ran into this please help. i am am also lookin for an aluminum driveshaft. and some pro star, doesnt matter if they r welds or jegs brand, any will do!

[Blue28]

Is the 12 bolt 2" narrower per side or total L&R ?
However many inche/s narrower per side 12bolt is, just subtract that from your backspacing.