On wood windows without casing, sill horns are based on the dimension beyond the frame:
Selecting standard sill horns would assume the application of BMC in the field. For WUCAs that would mean a 1-9/16” sill horn. For WUDHs it would mean a 1-5/16” sill horn. For WDG Rectangles it would mean a 1-1/2" sill horn. See detail below.