The3rd Earl Of Derby wrote:

No offence like.

I've had a think and all it takes is a bit of wood with increments
of half
inch steps at one end, increments of 1/8th steps in the middle and
1/16th
steps at the other end, marked on the wood.
Position the wood against the fence and move the fence near to the
router
bit in question and adjust the height.

And then comes the test cut, usually on a piece of scrap.

Lew