Some of the changes to the timetable, when it comes in, will effect unit utilisation as hinted at elsewhere. Also the number of maintenance spares will complicate things for anyone guessing in advance. Historically this has usually been around 5% of each fleet type, but can vary depending on fleet type overall numbers. As an example the present fleet (AFAIK) that is in service each day is:
455: 76 from 91. As mentioned above this was not originally planned to be so low, and no doubt will only be temporary until this December (they may also be putting some of this apparent slack to good use at the moment to help out with any shortfalls?). It could go up to 86 in theory which was the case many years ago, but despite their recent AC conversion they are getting older, so who knows how many they plan to use each day?
456: 20 from 24
458: 32 from 36
707: should end up 27 from 30. Yes, an odd 5 car diagram start is still in the mix (from Strawberry Hill), which includes 2S13.
With a new homogenous fleet, and a one (stock) type maintenance regime, it may be that overall a fewer number of vehicles/units will form the maintenance pool, thus a larger overall percentage for daily use. With the present situation can't necessarily take a 'maximum' formation rule as a guide, in the sense that on the SW network there have never to my knowledge, been enough units to make all services max formation all day, hence a degree of splitting/joining during the mid day period, primarily at Waterloo to ensure the correct formations are in the correct place for the evening peak. Eg: The odd 5 car 707 diagram in the evening is not the same one that starts out in the morning!