While there may seem to be an overwhelming number of input fields in the Saddle dialog box, there are really only 4 that are essential for properly defining the saddle as it pertains to fit and for positioning the saddle with respect to the rest of the bike.
Two dimensions that we may want to consider together are dimension L which defines the length of the saddle and dimension A which locates the low point along the top surface of the saddle. For saddles with a flat profile along the top, you may think dimension A shouldn't matter. However, this point is also where the rider sits. It's also where we locate various other dimensions such as saddle height and rider compartment.
The next two dimensions we want to consider are the horizontal dimension P and the vertical dimension Q. These dimensions locate the center of the rails with respect to the point defined by dimension A. In BikeCAD version 15.0 we now have an alternative to P and Q. Dimensions P2 and Q2 define the center of the saddle rails with respect to the nose of the saddle.