Word of caution here. The use of two opposing parabolas will get you into trouble. Dale is right. That is not the usual formula for an ellipse... rather, its something I used years ago doing some layout. Dont even wish to go there on a woodworking forum.
A circle is a special case of ellipse in which both centers are conincidental.
But who needs to remember all these goofy equations when you can just setup the micro fence. Using a micro fence makes this task brain dead and what the micro fence does not kill in terms of brain cells, jack daniels does the rest.
Had the dog not stopped to go to the bathroom, he would have caught the rabbit.