Help me confirm my measurements and back spacing thought

i have a 1970 Dart Swinger, 8 3/4 rear axle, dr diff big bolt pattern axles, and I installed a 3/4" offset kit.

When I measure from the drum surface to the outer wheelhouse lip it's about 4 7/8" and from the the drum surface to the spring I get 7".

My plan is to is to get a 15x8 rim and put on a set of 275 60 15 tires.

I'm thinking I need a 5" offset to make it work. Can someone chime in to make sure my math is correct.

Thanks