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Daniel,
520mm ID UTA should be fine, just keep your focuser board a little closer to the inboard edge. I made my UTA 502mm ID and 602mm OD and the height of the cage is 300mm.
With an 18" F5 primary and 3.1" secondary you can have up 330mm from the centre of the secondary to the focal point. The Feathertouch FTF2015BCR uses a 6mm base plate, 37mm focuser body and if you rack the focuser out 11mm to accommodate various eyepieces, you end up needing 54mm for that.
330mm - 260mm - 54mm = 16mm
So locating your focuser board so that its outside face is 16mm from the UTA inside face will give you 330mm as your "D2" which means you end up with a fully illuminated 1/2" field which is good for general use.
That should leave you a distance from your primary to the centre of the secondary of 1955mm or thereabouts.
One thing to check is how much does your secondary holder obscure the mirror? If the holder has a lip that cover part of the mirror you should check the actual clear minor axis measurement.
My calcs were based on the outer 1.5mm of the secondary being covered by the holder thus leaving a 3" minor axis....
I set my focuser board a fair way in so that I had easy access to the SIPS element to be able to remove it for collimation
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