Focal Plane problems

[PJO (Philip)]

I'm currently building a 10inch (254mm) F4 truss tube dobsonian telescope and have come to my first (and hopefully last) design problem.

The mirror is a SkyWatcher 10inch F4 mirror but I measured the focal length as being 1002.5mm making it aproximately F3.947, this means that with my current 46.5mm diagonal the focal plane is 11mm out from the edge of the 340mm upper end.
Here's where the problem lies though, (you've probably spotted it) where do I put my crayford style homemade focuser? The focuser is loosely base on Jerry Oltion's excellent design (see the link) used on his 12.5inch track ball scope, but I haven't actually made a final version of it yet. http://www.jerryoltion.com/12inchtrackball.htm
The upper end of the scope is 340mm wide so I have 40mm on the inside on either side of the light path to potentially put some of the focuser but is that enough? My bearings (from bunnings) are 19mm wide so should I put two on the inside and two on the outside? Or should I get a bigger secondary mirror; a 50mm mirror would be 25mm and a 52mm mirror would be 33mm out from the tube.
I haven't fixed my secondary to its support or fixed the focuser to anything yet, so I have time to change things. Also in case it matters, my spider is a single vane going across the diameter of the upper end.

Thanks to those who read this long post and what are your thoughts on it?

[Merlin66 (Ken)]

If you are considering imaging with this Dob you'll need more back focus space and a much larger diagonal.
Load the free Newt program and use the attached file to help. Looks like a 70mm diagonal....
https://stellafane.org/tm/newt-web/newt-web.html

[PJO (Philip)]

I wasn't planning on imaging, so I don't need heaps of back focus, but unless I use eyepieces other than Plossls I don't think its enough space anyway.
I would like to get better eyepieces in the future (my notable eyepieces are a 7mm Luminos, an old 2inch 32mm Meade UWA 70˚, and a vixen lv 8-24 zoom, other than that its mostly plossls and an Astromania 16mm UWA 80˚), but at the moment I need enough space for an eyepiece with its field lens 30-50mm up the eyepiece.
I have been using the Newt program but I was wondering if there was any other options other than a larger diagonal.
Also how much back focus from the tube would you recommend?

[PJO (Philip)]

Also it looks like a 70mm diagonal introduces vignetting on Newt. I have a 1.25 inch focuser tube because I don't really want the hassle of making/using an adapter. I have an adapter on my other scope, and its alright because I can put it in between the forks when I'm not using it, but I couldn't do that on a Dob.