Wheel dishing question

You can make a dishing tool out of thick corrugated cardboard. Just measure the distance on one side of the wheel then put the cardboard on the other side and see the difference in the distance.

Make it look like this, where the hashes are a ruler, the sides of the tool sit on the machined part of the rim:

Edit: ASCII art did not come out :(

yo can make a dishing tool out of courageted cardboard I have seen it If i find the link I will post.
You use a simple pointer device in the centre to measure the distance to the axle to compare with none dished side.

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