Mark, to determine the fuel at the gate before leaving is in fact pretty simple.
It involves a little work.
First I go to my favorite flight tracking site. Flightaware, check the airliners for the same flight. I will take the average time of the last week
as my flying time. Has to be the same aircraft. In my case 777-200LR
I only do flights >4hrs.
I do shorty training flights, to fine tune my CG to come over the threshold at the required attitude of 1 or 2 degrees nose up. From that
I work to perfect my "autoland to have very smooth touch down.
From that point manual landing are fairly easy. All my CS planes fly great.

Back to fuel.

Airliners and me do this.  200LR

Taxi   18 min at 66.6lbs per minute    1320lbs
Burn  flight time in min X 271.3lbs/min  crz .82
Arrival delay (ATC, Hold etc..) 36 min
Altn  distance varies -allow 20min per 80nm
Final reserve 30 min
No add on fuel or ballast

This become your " block fuel at the gate.
It is imperative you fly in real weather  I also check the box in FSX to obtain "wind aloft in custom weather.

It works !!  ;D

Following a straight in approach, you are expected to land with 21120lbs ;D  Fly high -the 200LR goes to FL390 to save fuel
The last step up has to provide a minimum of 20 min of crz before TD
to be feasable.
Step up every 1 1/2 hr or 2 to the max.

This guy needs 221725 of fuel to complete the flight.

If we want to duplicate the flights airliners do. We have to prepare like them.

Happy flying    :o

Raymond  Boeing or nothing  ;D
KFLL South Florida