How do you measure that accurately. Do I guess where the center of the wheel hub is?
When I do mine accurately I use the plastic top of a spray can which happens to be the same diameter as the rim's centre cap so I've marked the exact centre of that can top and when I put it in place of the rim cap it gives me a point to measure from which is level with the fender, so I get a very accurate measurement.
Or just put a piece of tape on your centre cap, mark the centre as best you can and use that. It will be better than doing it by eye.