If you want a true ellipse then get the formula out of machinery handbook, I'd scan a copy and post it but my scanner doesn't work right now. It ends up with 4 arcs, ends and top / bottom,, so it is going to run with I J or R values. I tried a cad snap ellipse and it didn't fit but used the formula and cut my part and it fit perfectly. The project was an adapter to the intake of the vette motor I had in my Jeep sold and gone now.