This diagram shows what happens when we rotate the flat 45°.  Real Flats don't stretch or shrink, but the required size does. All the stretching and shrinking takes place in the horizontal plane only.
In the next few pages I will calculate how much the flat needs to grow in order to catch all the Light Cone no matter where the flat is in the light path.  I will also calculate any resulting changes this would bring to the telescope's Clear Aperture.  You may be in for a surprise.
There is enough information in the diagram at the left to perform all the calculations.  Note that the shaded area is a single triangle that covers three regions in the Light Cone's Tail.