A good rule of thumb we use in real flying to estimate true airspeed is 2 percent per thousand feet of altitude.
For example, at 18000... 18 x 0.02 = 0.36 or 36 percent higher. 247 x 1.36 is 335 knots.
I use this formulae in ME to set the tanker speed. I choose an indicated airspeed to use in the tanker track, figure the TAS differential by 2% per thousand feet of altitude in the tanker track. Then multiply the indicated airspeed by the TAS differential and set the tanker speed to the result.
Kevin