Ranger offset/backspace

Im planning on running a 20X8.5 front and rear on a 99 ranger bagged/bodied. And I need to know what backspacing/offset is needed to clear the bedsides/frame...

with a 20x8.5 you will need a +35 offset for the rear to clear the bedsides on the stock axle if your runnin the 8.8 axle you'll need a 45 offset