19 Sqn and 92 Sqn in the Phantom FGR2 era were 10 Aircraft Established (AE) + 1 x In-Use Reserve (IUR). The 10 AE were funded for aircrew manning at 2:1, fuel, spares, flying hours etc - the IUR was not - but was usually the one on Minor / Minor * at 2nd-line in Aircraft Serving Flt (ASF).
For TACEVAL Part 2s, savvy EngOs would arrange for the 1 x IUR not to be in ASF and - if you were really lucky - arrange for the one coming out of Major at St Athan to arrive in RAFG but delay the one going away.
% Combat Availability - captured on the hour and divided by the total number of hours on TACEVAL Part 2 - was based on the 10 x AE. Hence how 92 Sqn managed to get 112% average on one TACEVAL I was involved in.
The Luftwaffe were even better off - a wing of F-104G had an AE of 2 x 18 AE squadrons but often was allocated 52 aircraft. This also allowed both the non-flying QRA(N) commitment to be held and 2nd- line maintenance to be carried out without affecting daily availability on the sqns.