A good rule of thumb to find the difference between RAS and TAS is approx 1.5% of TAS per 1000ft of altitude (assuming ISA temp).
For example, assuming a TAS of 250MPH, the difference between TAS and RAS at 15,000 would be
15 x 1.5% of 250 =
15 x 3.75 =
56MPH
So the RAS would be approx 250 - 56 = 194MPH
The big problem with this method is that you need to guess the TAS to start with, whereas the RAS is right in front of you! A bit of iteration is required.