to aaron chaim.
it is correct. the interval between RH and pesach varies.
the interval between pesach and RH can not vary. RH always comes out two days after pesach. IOTW pesach must come out two days BEFORE the FOLLOWING rh.
The controlling rule though, is lo ado rosh, which indirectly controls many other things about the calendar.
You can think of it your way if you like, but the calendar is always calculated by first figuring when RH must come out for this year and when it must come out for next year and working from there.