A question about field flatteners

[barx1963]

I have my new Stellarvue scope. It came with a dedicated field flattener that is designed to work with a DSLR. I also purchased a number of rings so I can adjust the focus distance for other cameras with different back focus distance.
My question is, how exact would one need to be with the back focus distance?
If it says 55mm, would 54mm still be ok?
I am guessing that it would depend upon the size of the sensor and the pixels?

Cheers

Malcolm

[Atmos]

Within a 1-2mm is generally considered okay, some flatteners are more forgiving than others. Some a very sensitive!