I keep forgetting the formula, so here it is for posterity.
In
PHP, it looks like this:
$foci =
round (
sqrt (
pow ($major_axis, 2) - pow ($minor_axis, 2)) ,4);
And here's a form to prove it:
Place nails 1.7321 units apart, string is 2 units long.
For a nice explanation of elliptical math and nomenclature, see
http://en.wikipedia.org/wiki/Ellipse