Help on an approach to backfocus

[AlistairW]

Hello,

I have a F5.9 refractor with a 478mm focal length. I recently bought a Hotech field flattener, which stated backfocus as 55mm. I had no problem in achieving the 55mm backfocus, but my subs all had stars streaking away from the middle of the frame. To me this has usually says my spacing is not sufficient. So I went to 57mm, and the subs were near identical. I was only taking 20s subs so there was little change of trailing etc. However, I have recently read some reviews where people with faster scopes than me have has to use 70mm backfocus. Is this really the case(?) - what would be a sensible approach to getting correct backfocus, as I don't really want to go incrementally from 55mm to 70mm in 0.5 delrin spacers?

Thanks

Alistair.