I think that’s what I did, I used a spirit level but I can’t remember whether I took the measurement from the wheel centre, rim or tyre edge. The question is though how far should they come out to look good before it’s too far?If the wheel face isn’t protruding from the tyre or the rim of the wheel, you can get a straight edge of some description across the wheel up towards the arch. Then measure the distance from the straight edge to the arch edge and use that number to determine how much further out you want to push the wheels.