Effects are kinda linear so what you want to do is compare two images with different back focus.
Plate solving the two images plus knowing the change in back focus can allow you to roughly calculate things. I did it a few years ago, there is a bit of guess work but I managed to get it accurate to within 1mm. I was doing something different though. The back focus of the reducer changed the amount of reduction so it wasn’t about correction but getting the most out of the reducer. Happened to be that when I got the most out of the reducer the correction would shocking
