Shouldn't be too difficult.
There's two things to watch out for, one is the shape of the texture map, the other is the shape of the faces on the texture map.

Examples :
If your texture map is (say) 600 wide by 300 high, then you need to make your ellipses twice as wide as they are high.

If a square face in a model is a rectangle on a square texture map, you have to match the height to width ratio of the ellipse to that of the rectangle on the texture map.

The most awkward situation to get things right in, is when the above two situations happen at the same time.