Ken,
It would work as the Filterwheel is 22mm leaving 4mm.
I think you're right, I'll sketch and see what it comes up with.
Section 8 / 9 of the ONAG Manual talks about Flatteners / Reducers and where place them and to be honest, it was way over complicated for me (I feel really dumb in this space sometimes).
I will go ahead and sketch as you suggest and see where that gets me.
Thanks for your patience with me Ken, much appreciated.
Pete