The secondary`s eccentricity of a Gregorian telescope is really pretty easy to calculate. The focus of the primary is one focus of the ellipse and the focus of the system is the other focus of the ellipse`and the secondary sits on the one of the sharp ends of the curve.
This is from the Ingalls ATM book Vol. 1 of the Willmann-Bell reissue which interestingly is the only place where I found any info on designing Gregorian telescopes. The other item in the chapter was to insure that the secondary is large enough to properly illuminate the full FOV at the focus point.