Here we see the Main Mirror's unubstructed light Cone again.  Only this time the pertinent dimensions are given so you can calculate the half angle "a" at the apex of the light cone. These relationships hold for all Newtonian Mirrors and Focal Lengths.
When the Flat is inserted into the Main Mirror's Light Cone this angle does not change, and can be used with the Radius of the Flat to calculate the "length of the Tail". Conversely, given the length of the Tail needed to work with a given Focuser, The radius of the Flat needed can be calculated.