For some activities it's best just to go by the hour.

For example, a few hours of mixing, some extra hours of tracking, drumming on your song, or if you want me to master a track....

For all projects:
extra time, loose hours, loose ends, I charge €44 per hour, ex VAT

If we book a full day of working together, that's 8 hours from 10:00-18:00, whether it's recording, mixing, producing - you get a deal with a flat fee of €296 ex BTW.
This comes down to €37 an hour.
We start at 10:00, have a lunch break at 13:00, then continue from 14:00-18:00.

Note: The last 30-45 min of your booked time we might need to do studio-cleanup, depending on the activities of the day!

Come and record a full EP on analogue tape in three days: you'll get my super deal: full package for €840 ex VAT.

 
This only works if you are well-prepped and super rehearsed!
On day one we track all the basics (drums, bass, guitars) and possibly vocals - as much live & at the same time as possible.
On day two we do vocals, record all extra layers and tie up any loose ends.
On Day 3 we mix and master.