The elliptical section is very nonstandard. The program uses a generic mathematical equation to approximate the ellipse sections that are produced by various manufactures. The primary reference for the ellipse equations used by this program is Engineering Mathematics Handbook, Jan J. Tuma, Second Edition, McGraw Hill, 1979.
Figure 6: Ellipse Dimensions
Where
The wetted perimeter is computed by iterating through the wedges until the correct wedge x/a dimension is found.