The direction of the rulings of a true developable surface are determined the the edge curves only, The user can decide what spacing to use, etc when displaying ruling lines but not the direction.

What don’t you like? Given the edge curves rulings will cross the transom. If the edge of the transom needs to be a straight ruling then the edge curves need to be altered.

I assume you are aware that while all developable surfaces are ruled surfaces, not all ruled surfaces are developable surfaces.

A developable surface has one principal curvature which is zero everywhere. The Curvature command calculates the magnitude and direction of the principal curvatures at a point on the surface, and displays a pair of circles which illustrate the principal curvatures. If a principal curvature is zero then the corresponding circle is a straight line.

Guassian curvature is the most common measure of how close a surface is to developable. That is probably because it is a scalar and easy to display. If one principal curvature is zero then the Guassian Curvature is zero and vice-versa. I prefer looking a principal curvature because I can see how close the surface is to having zero principal curvature in one direction.