Here are some links to a few more online calculators which may help with elliptic/circular arch dimensioning and layout. Scroll down the pages for the math and diagrams.

Circular Arch Calculator
Circle Center given Three Points on the Circle
Elliptic Arch given Major or Minor Axis
Elliptic Arch given two Arch Dimensions ... this might be what you are after
Ellipse Axes given two points on the Ellipse
Intersection of an Ellipse and a Line

Hope this helps. If you decide you want to go with a different conic section such as a parabola or an hyperbola I can post links for these curves as well. Or how about a sinusoid, cycloid or a catenary? smile

EDIT: MO, If you want to solve the arc lengths on an ellipse between any two points and have a programmable graphing calculator with an integration function check out these example formulas .

Hip-Valley Roof Framing and Joinery Math