I got a déjà-vu feeling on this one as soon as I read the first line…
Boy, it even has the same title 
:
http://discourse.mcneel.com/t/elipse-through-points-no-command/9843/1
And then there are these - all by Steve:
http://discourse.mcneel.com/t/establishing-major-axis-of-an-elipse-type-closed-curve-how/
http://discourse.mcneel.com/t/where-is-elipse-from-centre-tangent-curve-option/
http://discourse.mcneel.com/t/how-to-find-centre-of-an-elipse/
Feel like we are going in circles once in a while - errr… make that ellipses.