Question about backspacing.

[texasbred]

I have been trying to figure out what backspacing I will need for my wheels after its lowered. I have talked to Bogart's and they are make me some 18x7's for the front, and 18x10 for the rear if they will fit. They said the maximum backspacing would be 6.5". I'll want to run the MT Sportsman S/R in a 30x12R18, which is a 29.9" tire. I'll be lowered only 4" in the rear. I don't see why they wouldn't fit.

I just can't figure out the backspacing for the front or the rear, which aggrevates me because I use math and huge algebra formulas everyday at work.

So any backspacing suggestions for the front and rear would be great.

Thanks

The original plan was to strip the plastic chrome off my factory 20's and have the wheels powdercoated, but I'm having trouble finding a decent DR size to run on the 20" wheels. I figured the 275/40R20 would look small on a truck dropped only 2/4.