What is a "safe" distance between the inside of the tire and outside of the frame rail ?

I need to "move" my tires IN,,,they are too close to the lip of the quarter panel.....and the tire is rubbing/hitting the lip of the quarter panel

I should be able to do this with changing the backspacing on the rims,,,,,

Right now,,,I have a Weld Prostar 12 x 15 rim,,,with 6.5" of backspacing....

Right now,,I have 2" of "room" between the inside of the tire,,,to the outside of the frame....(see pic")

If i was to change to 7.5" of backspacing,,,that would leave me 1" on the inside of the tire to frame....is that ok...too close ... just right....??

Trying to avoid shortening the rear end to gain the clearance