Good stuff Dennis. Thanks for all the info.
On the paper the CN-212 at F12.4 gives me 2630mm FL and an image scale with the QHY9 of 0.42 with a field of 23.4'x17.4'. Because like you I'm further at the back those figures will probably be on the low side.
I have a reducer to bring it to F9.9 but that's another element that takes backfocus.
That's a really good shot up close. Still not many gaps between stars even at that FL.