Need help with backspacing

Hey guys, looking to get some 18s on the Dart (I know I know, not everyone’s cup) and have some questions on backspacing.

Between my wheel mounting surface and already rolled fender lip I have 4” and then from WMS to spring I have 6”. Total of 11”. Largest tire I could run would be a 245/40/18 at 9.8” total width which would give me 1/2” clearance on each side right? This is with hubs on a 8 3/4, I would one day like to run rotors, I read add 3/8” to the mount in surface?

From my math I am coming up with a 9” wheel I would need 5.25” backspace to center in the 11” space and can run a 1/4” spacer until I get the discs?

I have a 1/2” spring offset kit I’m debating using but don’t really feel like welding on the 8 3/4. Would that let me bump to a 255 tire?