A previous post gives EMT 58 units for their local services. 7 of these are expected to be stopped for their normal, scheduled maintenance. So a maximum of 51 units can be expected to be available for service on a given day. 51 out of 58 is 87% availability - not bad for units of this age I would have thought.
The off-peak timetable can be operated with 37 units.
The peak timetable can be operated with 39 units.
Both figures being the absolute minimum necessary to cover all timetabled passenger services but ignoring any strengthening for capacity purposes.
Doubling up the Nottingham to Liverpool workings, requires a further 7 units (6 if stepped up at Nottingham), taking the total requirement to 46 out of the planned availability of 51 units. I assume the final 5 are also out in service, strengthening other services, whenever they are available.
So, if EMT are up to 5 units down, the Liverpools can all run as 4 car, if EMT are more than 5 units down, somewhere a service has to suffer.
Given that they have parallel services at the same, if not higher frequency throughout, I would suggest that it is better for Liverpool to Nottingham services to be short formed than to cancel a service elsewhere on the sprawling EMT network where there is no parallel alternative provision.
If the train fairy is to magic up some spare units, as part of her grand rolling stock cascade, just how many should EMT get and where should they place them. 1 each at Crewe, Derby, Liverpool, Manchester, Sheffield, Nottingham, Lincoln, Grimsby, Peterborough and Norwich? That way there will be a unit to slip into service should any service interruptions or unit failures occur. They could all be hot standbys, with a crew ready and waiting.