I would say no such thing as a HEMS variation. There is a scheme your operation has agreed with the CAA which you might use for HEMS but there's not really a selection of off the shelf ready made HEMS options in CAP 371 (current UK helicopter FTL guidance - written before HEMS was a thing). Any variation is done by asking the CAA.
Also your manual and SOP should make it clear how much discretion can be used and how it can be used. The decision to use 1 or 3 hrs should also be based on some guidance in the SOP based on crew fatigue and operational need.
As examples of what manuals should cover, disctetion must be never be rostered (eg your shift today is 15 hrs to cover Dave who has a medical appointment). It should only be used if a particular HEMS task takes you over your shift end time.
But your procedures should also cover a few fairly common scenarios.
- What if the job comes in before shift end but you won't be able to lift from base until after shift end?
- What if you are already in discretion and you get a job on the way back to base?
- What if you have started to get the aircraft back in the shed, removed a few items of kit and then you get a job?
There is scope to amend an FTL scheme to suit. If you don't like the scheme as it's too permissive (eg 3 he's discretion is too long), change it through an NPA