Copied from the "cup final return travel issue" thread for info. East side allocation makes travel back from Wembley Park to Euston Square easier, as the majority of Watford fans will walk west to Wembley Central for the Overground.
In terms of the return trains on May 18th, it's broadly the same plan as on Saturday, although the route via the Potteries is open up until the time of the normal last trains, so one of the reliefs will serve Stoke and Macc.
20.20 Euston to Manchester will run as normal
20.54 Euston to Manchester unadvertised relief calling at Wilmslow and Stockport (will not run if match goes to extra time and / or penalties)
21.00 Euston to Manchester will run as normal
21.20 Euston to Manchester advertised relief calling at Stoke, Macclesfield and Stockport
21.24 Euston to Manchester advertised relief calling at Stafford, Crewe, Wilmslow and Stockport
21.46 Euston to Manchester unadvertised relief (will only run if match goes to extra time and / or penalties) calling at Wilmslow and Stockport.
The line will close after the passage of the 21.46 (if it is required to operate).
All the above is subject to validation, but I'd expect the advertised services to appear in public systems by this time next week.