Seems to me that the pinwheel portion is 2 rails wide? Is that right?
Each 2" rail will finish at 1.5", so each HST patch of the pinwheel will finish at 1.5" (the entire pinwheel will be 3" finished) So you add 7/8" to the finished size of 1.5", which comes to 2 3/8". Since we want a little wiggle room, round up to 2.5", sew your HST and trim.
Make a sample to double check!