It's supposed to move, when measuring off an edge the clip slides out as it hangs over the edge and will move the thickness of the clip to keep measurement correct. Can you stand the wheel up and put the 1" mark on the backside and read the measurement easier? Looks 7/8" in the pic, if it is 7/8" you will need to go with the 3/4". Also as kuda mentioned 1 1/2 times the stud dia. Is all thats necessary for thread engagement. If your studs stick out at least flush with the wheel than 3/4" nuts will be plenty
