You're welcome on this forum as a sim pilot, with a genuine curiosity -
as long as you are honest enough to admit you are a sim pilot 
People are only put down if they attempt to claim experience they don't have.
Your flight (I'm on the 737, forgive the differences):
15000 lbs trip
+ 750 lbs contingency
+ 1500 Alternate
+ 3000 Final reserve (30 mins, in my company)
Gives a Block fuel of 20250 lbs (which is what you would tell the refueller to load).
You would expect to arrive at your destination with 5250 lbs, a bit more with short cuts, a bit less with weather manoeuvring.
You would set your FMC fuel alarm at 4500lbs - the minimum to divert to the alternate. The aircraft low fuel alert in the 737 triggers at either 907kg/tank or 453 kg/tank (depending on the modification). Say it's 907kg (times 2, times 2.2 lbs/kg =) 3990 lbs.
Now say you arrive, and are given unexpected holding. Once the FMC predicts the fuel
at arrival at the runway is below 4500 lbs, it will trigger a "Using Reserve Fuel" message. For this to be accurate, you must have correctly programmed the hold (including the Expected Approach Time), and expected vectoring after the hold. At this point (actually well before this point!) a decision has to be made about which airport you must commit to - commit to destination, or commit to a diversion on minimum fuel. At my company an Air Safety Report is also required - so we are now into having to complete extra paperwork (something every pilot tries to avoid!)
Let's say you decide to commit to the destination. I would reset the FMC alert to "final reserve + 100 or 200 kg."
Continue to hold, and at 3990 lbs the aircraft low fuel level will annunciate - this is a QRH procedure to ensure all of the remaining fuel can be used - turning on pumps, opening crossfeeds and the like.
Continue to hold, and the FMC annunciates "Using Reserve Fuel" again!
Again, given correct FMC route programming, it now looks like you
may use your final reserve (may, due to the added 100/200kg above) - this warning is a mandatory "PAN PAN PAN" urgency call to ATC for priority vectoring.
Continue to hold, or get poor vectoring, and it now is
definite you will use final reserve fuel - this is a mandatory "MAYDAY" call - and a trigger for me to first stress in the strongest possible terms I need a direct route to an immediate approach, or in the final instance, to force the issue by flying a direct route, with a general broadcast to other traffic (ie ignore ATC).