How much back spacing do you need to clear the brakes on a 15in rim?

 

I don't think back spacing is an issue with clearing the brakes. I was told a steel rim in a 15 will clear the brakes bc the inside of the rim is thinner than an alloy rim which is much thicker. So if you have a 15 in steel rim it should clear the brakes. Where an alloy will not. I do know 4 1/2 in back spacing seems to be the rule to clear your tires of the inner fender wells and shocks.

Someone pls correct me if this is incorrect.