OK I think I finally managed to solve it.
I had elongated starts on the top left, bottom left, hardly any on the bottom and top right.
So I dialed in the backfocus and my field flat went from 7.1 to around 11.1 and now I have around 2% tilt and 16% curvature. Stars on the top left and top right are much much better. Ever so slight elongation but would never notice it.
So total backfocus I have now is 66.7mm
Time to tighten it all down and start imaging!
Thank you
