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Paul,
You just need to know/ measure the focal length of the reducer.
Once you have this you can use the simple lens equation to find the "back focus" which depends on the distance from the prime focus to the reducer.
1/F = 1/U + 1/V
where F is the fl of the reducer
U is distance from reducer to prime focus
V is the distance from the reducer to the new focus.
and V/U = magnification
HTH
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