Tubbing the firewall on a shitbox

I dont have my 22's yet but wanna get the firewall tubbed while I am waiting. Right mow I have stock wheels with a 245/75/16 tires which I believe are real close to the overall diameter but not sure how wide to make the tubs. I am working on the pass side first, anybody know how far from the outer edge toward the center to cut? I will be running 265/35/22's amd eventually 2" longer arms with 3/4 ton hubs. Any help would be appreciated

ill try to run by the shop tomorrow and measure mine and possibly send you a pic of mine. i over did mine so i can turn a lil when laid out. but im runnin the exact same setup with the wheels tires longer arms and 3/4 hubs.

i think i did 12'' but im running 24'' and i want to turn low i got 3/4 hub with longer arms. i usually do 9-10'' on non duallies obs.