Rear end location wrong? IM STUMPED!!!

that's dependent on a lot of things.

are you trying to squeeze max tire in there? are you trying to buy new rims? are you trying to buy new tires? what do you currently have? can you go more without moving the spring?

unless you're following a recipe there's no one single answer. ,

and even then, when you start to push the edges of what will fit at max, max, you can get that tolerance stack that will throw a wrench into everything.
There are crager (sp) 14" rims w/ 225/60r14 front and 245/60r14 back tires on now. I'm hoping I can go to a 15 or 16 diameter rim with a 225/50r15 or 16 on the front and 255/50r15 or16 on the back with a 7in wide rim. My thought is less bulge in the tire may help with clearance and I'm not a big fan of the taller tires like my pops was.
I need to figure out the backspacing on the rims and plus or minus them correct?