tire + rim size shortend diff

Housing flange. Just need backspace for weld drag lite with a good front side space. Have a 15x10 with 13.5 wide 28.5 tall tire bs 5 1/2. Can stack 3 ,1/4 spacers 3/4 over all to get tires to tuck fender and 2+ " to leafs Just unsure of offset??