Technically, only those at the rank of Corporal or above will get prior service pay, at the rank which they were, and with actual time (all of Class B, and I think 1/3 Class A days) towards their IPC. As it stands now, I've heard of many privates being given the same privilege.
After 3 years in the reserves, being a Corporal for 2 of those, I was given Cpl. Basic pay with 273 days towards Cpl. 1. Your Mileage May Vary, but given that you have both prior Reg and Reserve Experience, I would expect to be making REG Force NCM Pay, moving the Pay Category D on commissioning.
Hope this helps.
***Now noticing that this is several weeks old.***