Did a test to see how to use Bezier curves to approximate a circle/ellipse (using information from this page, and discovered that at higher ratios, an ellipse has... problems. Adjust the length of 'Line_C2_C3' in the attached file to see what I mean. (FWIW, to automatically adjust the control points of a BC, you just need to multiply the appropriate radius by 0.551915024494, as seen in the options for the bezier curve in the example I provided.) The position of C2 can also be changed to show off-center curves.
Hope this helps!