In most aircraft you will find a performance manual, it should show the TAS for a specific altitude, otherwise you can use something simple like the Pilot Wizz APP for Iphone/Ipad, or on a website like
Calculate your True Airspeed (TAS) [Aviation Rules of Thumb] - Flightsim Aviation Zone
You said that you knew the theory,
that it depends on pressure, temp, and winds..... Are you sure about this? Look it up again, especially the part about wind.
For the purposes of your flt sim, interpolate the wind, its up to you to decide on the number of interpolations that you want to do.
Good Luck
Mutt