That depends on the intercept distance (secondary to focal plane) and the size of the field in your lowest power eyepiece.
One way to use Mel's calculator is to enter the fieldstop of your lowest power eyepiece in the illuminated field size (e.g. 31mm Nagler = 1.65"), and select the diagonal size that doesn't descend below -0.4 magnitudes at the edge of the field shown. I think it's a good logic to assume that the telescope must take the field stop of the Nagler 31mm without any problems. Thanks.
As to what the intercept distance is, it is the sum of:
--clearance to inner tube wall
--thickness of tube
--height of base of focuser off the tube
--height of focuser racked all the way in
--1" additional to focal plane to allow things like barlows and coma correctors to come to focus and have nearsighted friends look through the scope.
So, take a 12" scope. It is hard to see an intercept distance in a properly designed scope will be less than about 114mm.
As a rule of thumb, I always add 4.5" (114mm) to the mirror radius.
A minimum of 19mm clearance between mirror and tube to keep upper opening from vignetting the incoming light and allow ventilation of the tube without significant air currents in the optical path.
A usual 25mm between the base of the focuser and the I.D. of the upper tube assembly to allow for the focuser board and ring and to keep the drawtube out of the optical path of the scope.
A low profile focuser 45mm tall. In reality, many are taller.
A focal plane clearance of 25mm over the focuser.
Thanks for that!
If any of those numbers is larger than that, then you will add length to the intercept distance. If the scope is used for photography, the secondary is larger (as it is in your example).
A purely visual 12" f/5 scope would be fine with a much smaller 66mm secondary, but that would assume a shorter intercept distance. Right, I'll inspect the numbers of the GSO tube tonight to see about the precise intercept distance with the 70mm and 80mm, just for the fun.
So, plugging in the numbers, and keeping the edge of field light loss to < 0.4 magnitudes,