From memory, when materiel and financial provisioning was done properly (scrapped in 1987), the Long Term Costings Permanent Instructions (not annual assumptions) stated that the Engineering Pool (aircraft in maintenance beyond 1st Line) should not exceed 13% of the fleet. You then added Training, IR and IR6 aircraft to that figure and what was left was, theoretically, available to front line, serviceable. It was the job of the "Provisioning Authority" (an old name for what was later split into Requirements Manager and ILS Manager) to maintain the figure, in that he (a civilian) was accountable to the 2 Star. The question that should be asked is what the equivalent Instructions require these days - if there even are any.
Obviously, in a relatively small fleet, and if there is a lot of initial training going on, the final figure may seem on the low side. Also, one aircraft going u/s can make a large percentage difference.
Beagle - -re your valid point about 8.33 spacing, in about 1995 OR issued a list of aircraft that would never need or get it. Many are still in service, while some who got it are not. That OR officer later became the IPTL of an aircraft that didn't get it and had the embarrassment of seeking an overturn of his own ruling. Not helped by the aircraft suffering break through every time it went near an airport (as predicted).
In fact, as the project office assumed the regulations would be implemented, it cost more to scrap the 8.33 design and revert to an old design. That's where the money goes folks.