Within reason on the vanilla C925 (As in not an Edge and not using a reducer/corrector) the exact camera spacing is less important. If you apply too much spacing then you will have to move the mirror further up the tube and might introduce vignetting from the baffle tube cutting light off. Another effect is that if you change the distance between the mirrors, you actually change the effective focal length somewhat. I would try it with the camera mounted as close as is practicable to the rear cell and see how it looks, but the focus knob will need quite a bit of twirling to get in from 90mm to say 45mm.
What I can't quite work out though is the description of vignetting making the corners brighter, vignetting should see the light levels fall off as you get further off center in the image. So long as vignetting is not severe you should be able to largely correct it with flat frames though.
I am using a C925 (Which used to be a CPC925 until I removed it from the fork mount) with a Celestron 0.63 reducer/corrector and an Astronomik L3 luminance filter (With resolved a huge reflection issue I used to have) and a ZWO ASI294MC pro. It is producing results the really are pleasing for what was supposed to be a visual instrument. This is one I am still working on the processing of from my C925.
https://www.astrobin.com/full/v74z82/0/?nc=user