Back focus ZWO asi 071

[GyCx]

Hi All.

I have a ZWO asi071mc pro and will be using it on a C11 with a 6.3 focal reducer I have been advised that i will need 85mm BF from the back lens of the reducer, the issue is if I use the 85 mm measurement the vignetting is massively pronounced compared to a shorter BF length. Can someone point me in the right direction to a formula  that I could use to work out the BF myself. 

Thanks.

Edited December 16, 2020 by GyCx
misspell