I primarily use TOPCAT with the NGX, and it provides a wealth of details. First off, for my VA I simply get a passenger and baggage number. From there, I input these numbers and it gives me an optimum payload distribution (how much to split the baggage between FWD and AFT CARGO HLDS, how many passengers in first/coach class).
I then feed it the airports and flight number, upon which it will give you an approx distance, and also a very approximate fuel load for that flight (I say approx, it's never actually been outside 1000kg variance with my 737 fuel planner app, so it isn't bad at all).
Now that the app has a close idea of your take off and zero fuel weight, you then tell it which runways at the airport you will use (as an added bonus, you can download the airports METAR, this is input and the app will 'best guess' the runways in use). This includes length of the runway etc. From this, it gives you an optimum take-off configuration, including derated and assumed temp (flex) takeoff parameters.
For example, I could be doing a short hop from Heathrow to Paris, so I'll be very light on fuel. This, combined with the long length of runways at Heathrow means I'll probably get given a F1/TO-2/60C takeoff parameter, which pads out to:
* Flaps 1
* Takeoff-2 (Derated; in the NGX-800's case, this derates max takeoff power down from 26k to 22k)
* Assumed 60C temp (reduces takeoff N1 even further)
Or if I'm heavy and with a shorter runway, it may even recommend F10/TO/35, which means Flaps-10, no de-rate, and only a slight reduction from the assumed temperature
These are worked so that you have enough runway available to stop from just before V1, and also derate your takeoff as much as possible to save engine wear.
It also goes further in that you can include Packs ON/OFF, Anti-ice ON/OFF take offs, and in the MD11's case with it's Dial-A-Flap capability, a recommended take off flap dial setting.
It's very easy to use, and adds another level of realism to your flight.