[Kunama]

The F ratio given for a reflector is the focal ratio of the primary mirror not the T ratio of the completed telescope.
As the OP stated, the t ratio will always calculate to 'longer' than the f ratio in an obstructed system.
The obstruction is often expressed in terms of a % of the aperture diameter when in reality it should be a % of the area of the aperture.

Secondary size depends on how far from the primary one wants the secondary to be. To minimize the size of the secondary it is placed as far as possible from the primary while still allowing for the full clear aperture and an allowance for tube currents, focuser height etc.
eg. A 10" F5 will need a minimum UTA diameter of 12" and depending on the focuser used, the focus point will be about 9" from the optical axis of the primary, so the centre point of the secondary will be 41" above the surface of the primary. You can then calculate the minor axis of the secondary.
That's my guess anyways....