I work in Poly/Cotton on a daily basis and I have found that the deeper your pleat depth is the better. If you have the fabric, make your pleats as deep as you can.
My FK's use a pleat ratio of; Cargo = 1.5"reveal/3.75"depth=9"Sett. Casual = 1"reveal/3"depth=7"Sett. Dress = 1"reveal/3.5"depth=8"Sett.
My deep pleats under the aprons are 1/2 the apron width. For you this would equal 10.25" deep. The reason for this is to allow for ample fabric to allow the aprons to fall between the knees when sitting.

Remember, Poly/Cotton does not move like wool. You need as much pleat depth as you can manage given the amount of fabric on hand.