In your case as an FO, P1/US or PIC/US are the legs that you fly, i.e acting as the PF. Really all it means is Pilot in Command Under Supervision. P2 is all the time you're acting as the PNF. You're going to have to go through all your SIC time and figure out which legs you flew and log them as P1/US and all the legs you didn't make up your P2 time.
