then that underlines what a risky endeavour PS is.. Pushing the envelope a little too close for comfort.
That is not how it works. They would have factored all that at the point of origin before takeoff
There are different types of "fuel"
1) Taxi fuel - fuel used prior to take off getting to the runway - which may need to be increased depending on local ground delays
2) Trip fuel - takeoff, climb, cruise, descent, approach, landing, and taxi to gate.
3) Alternate fuel - after trying to land, fuel required from the missed approach point at destination, climb, cruise, approach to landing at the alternate
4) Contingency fuel - fuel required for additional fuel consumption due to route changes, headwinds. I understand there is a minimum +5% on top of trip fuel
5) Final reserve fuel - the legally required minimum fuel required to fly for 30 minutes at the destination or alternate. Aircraft should have this fuel after landing and taxxiing to gate.
6) Centre of Gravity fuel - fuel may be necessary to maintain CoG and usually cannot be burned during flight depending on CoG envelope.
7) Additional fuel - the pilot can add extra on top of all that - such as for holding at an airport known to make aircraft hold.
Then add it all up and see whether it will fit in the fuel tanks and not exceed max take off weight. If take off weight is = MTOW but more fuel is required, then payload including passengers will need to be offloaded. Or plan to land at an intermediate airport to uplift more fuel. The problem when PS is that any landing at an intermediate before the destination will likely also mean the pilots will be going to a hotel.