The powers that decide were told what the establishment should be for each ship type. The idea was that if you added a ship type then you added the agreed crew strengths to the Establishment plus necessary Sea Going replacements in the knock on effect. In the Draft for the New Naval Structure for Naval base. Ships, Dockyard and HQ the Strength was AFAIK in excess of 1200 for, at the time, a 6 ship Fleet. It is now 9 ships without stipulated manpower.
I accept Bravo 20 remarks about PDA aka SGA and agree it should be a Tax Credit or Free of tax.
Notice was tiered with a ship varying from maximum agreed notice in the Basin, to standard operational notice at the OW. Once out on SO the ship remained at Immediate notice throughout her patrol duration. Notice could also be meaningless as a laid-up ship could be brought to 72 hours notice and rushed out with re-calls and replacements using chaos theory.