Try this little program, it may be helpful for optimising the mechanical parts dimensions.
http://www.dalekeller.net/ATM/newton...t/newtsoft.htm.
What you are doing is OK to avoid vignetting at back end and works for simple eyepieces (single lens design), however, some eyepieces have focal point well inside between the glass elements. Also as far as focal point is concerned, it should be sufficiently outside the tube so that, so for example, you should be able to reach it with the camera sensor, which in most cases is 45.5mm or a bit more from the camera flange (plus you have to allow for focuser length and focussing path).
The general design approach should be to have focal point as far out as possible, without vignetting (blocking the light cone).
I have 10" F5.6, tube is 33cm dia, secondary is 2.18" and focal point is 78mm outside the outer diameter of the tube. Originally I designed it to be 103mm, but later I moved the mirror 25mm backwards to avoid vignetting for 22mm camera sensor (and 1.25" focuser tube). Now I have a slight problem with barlow, so I will have to shorten the focuser for 10~12mm to be able to focus with some short eyepieces.
For the camera (Canon 400D), I will have to use separate, low profile focuser (30mm long, measured from the surface of the tube), this is already drawn in AutoCad and waiting to be made.
Also, be aware that the secondary mirror must be slightly offset from optical axis (away from eyepiece) to fully accommodate the light cone from mirror, in your case it will be ~2mm.