hex0rz
+1y
Well, since you have eveything off, find center point of the fram between the control arms. From there use the overall diamater of your tire and divide that in half. From the center point of your mark on the frame, (say OD tire is 26 inches, that would be 13 inch mark) go to the left and mark where your tire edge would be and do on the right.
Now you basically have your tire diameter marked on the frame from the center point of the frame rails. Then measure the width of your tire and go in from the fender. I would say add a little onto this amount, or test fit if you want for a tight fit. This will give you a mark of where the back of the tire sits in the well.
That although might be a big cut for you cause I have no idea if your tires are going to tuch in the well more than halfway...
Hope that helps?