Hello glc100o,
it's easy, but as you describe it here, it DOES NOT work.
Take a look at the GSX manual, page 17 and page 26.
But I will help you:
1) check your GSX settings! Open, while the simulator is running, the GSX setup. Addons -> GSX -> settings -> simulation.
2) The options "Always Refuel Progressively" and "Detect custom aircraft system refueling" must be activated. (Page 26/GSX manual).
That was it, considering GSX, too.
Regarding flight planning and refueling. It does not matter WHEN your plane is refueled and has no influence on your flight planning.
The correct amount of fuel must be loaded only BEFORE you make your input to the FMC (starting with "Position"). Why? Well, because your FMC is a computer that can only work with the right data.
For the tanker to work properly, you need to do the following:
1) open your FMC
2) go to FS-ACTIONS -> FUEL
3) LSK1 L shows you the current fuel weight (the fuel that will be pre-loaded at the start of the plane) in kg or gal.
4) depending on the operating condition (cold & dark or other) you call, note, that the APU need some fuel to run.
5) type a small amount (gal or kg) - I use kg and a minimum quantity of 300kg - so "300", in your FMC scratchpad.
6) You now see "300" in the bottom line of the FMC.
7) tap on LSK1 L and this value will be transferred. So your plane currently only has 300 kg of fuel (the same applies for gal).
So you're ready for the tanker, but stay in the FMC-Fuel menu.
8 ) Call the tanker via the GSX menu. While waiting for the tanker, type in your desired amount of fuel - the amount you have in your planning - into the FMC scratchpad. For example, 8500 kg, so "8500". These 8500 now appear in the bottom line of the FMC.
Then you wait until the tanker arrives.
9) the tanker arrives, positions itself under the wing and connects the filler neck. After that, you hear a beep and a gsx line appears, asking you to announce the volume.
10) tap on LSK1 L in the FMC and your set amount - 8500 in scratchpad - will be transferred to this line. the fill quantity now appears in LSK1 L "8499" (total kgs or gal). This minimal difference comes from the fuel consumption of the running APU.
11) With this step you have entered the required data for further work on the FMC. Now you can leave the fuel menu and transfer the flight plan to the FMC and make all calculations and settings through the FMC - all correct.
But what does the tanker do?
The tanker fills the plane with the fuel-difference between existing - 300kg - and needed - 8500kg - fuel. So 8200 kg. You can see that on the running counter (animation) of the tanker - on the right side of it. Again and again, a gsx line is displayed, which indicates the already filled quantity.
When the refueling process is completed, the tanker leaves the aircraft.
Important! The filling is just a simulation and has NOTHING to do with the function of the FMC! There, it is sufficient to enter the correct amount of fuel, regardless of whether the tanker is finished with the filling or not!
I know it sounds complicated and a lot of work. But, if you know how, you only have to enter two digits and execute two mouse clicks. A "work" of a maximum of 10 seconds.
And, why does not it work for you? Well, you can not fill a full tank without first emptying it ...
I hope I could help you.