I hope I am not being shick but I am confused by the under apron pleat. I have worked out my sizing 8" over apron and 10 x 3"pleats for a 38" rump. My under apron will be probably 12" on the other end, but what do I do with the 'under apron pleat' fabric that is next to the over apron? Is the over apron double thickness or something?