Need Help Measuring Backspacing - With a Pic

[AU Doc]

I feel like I'm missing something on my backspacing measurement. When I check the front at full lock, I'm getting about 2-3/4"" between the mounting plane and the frame. I'm measuring at about 13" from the hub centerline. I'm guessing a 245/40/18 is just shy of 26" diameter. I was planning to run an 18x8 up front, and I wanted to verify the 4.75" backspacing that is typically recommended.

It looks like the 15" rallye's I have on the car now have been rubbing, and I think they're a 7" wide wheel.

Is this measurement correct? Am I overlooking something obvious here?