Calculated Field to Manage Holiday Entitlement

I have been asked if we can monitor a teams' holiday entitlement via Workfront. I had the idea of creating a calendar and using the Time Off option as a filter which has worked fine when I tested.

The next ask is that Workfront 'manages' the allocation per person, e.g. 25 days per year, and displays the remaining leave they have left. I created a User custom form and added a field for their total entitlement and know I will need a calculated field to reduce that total number when they book holiday but I then don't know the best way of recording this, e.g another field on the form where the user/manager enters the days requested so the calculated field subtracts this - I wondered if there was an automated way of doing this if possible, so any suggestions would be gratefully received!