Mud Rims How Much Backspacing Do I Need On My Rims To Be Able To Fit Some 33x12.50x15 Mud Tires On My 91 Jeep Cherokee?

How much backspacing do i need on my rims to be able to fit some 33x12.50x15 Mud tires on my 91 jeep cherokee? - mud rims

This is a 4DR, has a 4 "lift and I cut the windshield, Bushwacker flares. For the population now has 15" tires. I am interested in qualifying for 15x8 wheels.