On the Stitch and Glue that I modified (in CAD) I used circular arcs through three points at each station. Th three points are where the station plane intersects left gunwale, the peak of the deck, and the right gunwale. By doing this you will increase the volume under the deck because the circle segment circumscribes the triangle it replaces. You can click the link below to see what I'm talking about.

> The deck is peaked. You may round it over if you choose. Just
> use a consistent method (such as a spline) for creating the curve
> to get a fair deck. Note that this will lower the deck, but there
> should still be enough room after rounding.