Backspacing on rims?

[Gremcat]

I want to get a set of rims for the truck. I am wondering what backspacing on a 15" rim I need to run 33"-35" tires with a wider less tipsy stance. My 13 year old will be learning to drive it on the farm and I don't need him rolling it. I am planning on putting a spare 31 BFG ATKO on it for now. Just got a spare carrier and want to get the rims for the tires I want instead of sourcing a single matching rim now for rims I don't plan to keep if that makes sense.