I have drawn the Major Axis of this diagram in two colors. The Blue portion on the right, is the short side, calculated as side c' above. The Black portion is the long side calculated as side c above. Note that the short side of the Major Axis falls short of reaching the ellipse's geometric center. The long side makes up for this by passing through the ellipse's geometric center to meet the short side at the point where the Main Mirror's Axis of Symmetry intersected the flat's Major Axis in the 45° position.
The distance between this point and the ellipse's geometric center along the Major Axis of the ellipse, is called the Offset.  This is a property of telescope Flats, not of ellipses in general.  The Flat's elliptical properties such as its Focal Points and its eccentricity, are shown in the diagram.