If it is indicated airspeed in a Hurricane you subtract 4 mph from your IAS to get CAS and 6 mph for compressibility effects to get 225 EAS.
225mph EAS * .869 = 195.525 KEAS
EAS * SMOE = TAS
195.525 KEAS * 1.27105 @ 15500 feet = 248.5KTAS
248.5KTAS * 1.15 = 285.8mph TAS
However that is all nonsense as the conditions in the summer of 1940 were not a standard day.
Your density altitude is actually much higher than standard in the game. That means you will see reduced Indicated airspeeds over the data converted to standard at the same altitude. Your TAS will be higher because the Standard Means Of Evaluation increases with altitude.
To calculate for the actual conditions in your game you need some atmospheric information:
Temperature
Pressure
density
dewpoint spread
At a minimum you can need pressure and temperature to approximate the density altitude.
You should be able to get both from aircraft cockpit instruments. Many airplanes have an OAT or outside temperature gauge and you have to set your altimeter to the pressure.