These figures (or targets) were set in the Permanent Long Term Costings Instructions - (mid-80s terminology when I used them). They determined just about everything that cost money. I can't speak directly for ships, but one was allowed to assume 14% of aircraft in the 'repair pool' at any given time (excluding a/c in SBM/SUM). Aircraft and ships were directly related, as the latter required aircraft, so a similar rule must have applied. As we only had 3 CVS, that would be rounded up to 33%. If aircraft rose above 14%, the Admiral would wander down from the 5th floor and ask the question. One assumes that's why MoD needs so many Stars nowadays!
At an equipment level, individual staff officers (now called Requirements Managers) applied (by hand, no computers then) a simple formula called STOCKCAL, which set similar targets for 1st and 2nd Line Recovery rates (10 and 80% respectively). Rule of thumb was, if R2 fell to 50%, robbery was inevitable. If you didn't get this right, you were summoned to the 5th floor. In 1988, as a savings measure, this became 8 and 68%, which left the RM less leeway and robbery more likely. It would seem this is even lower now, although probably not in the permanent instructions. By the way, these instructions were what set out the Requirement Manager's unique authority to use engineering judgement to overrule annual (not permanent) LTC instructions.