To use an E6-B for compressibility problems you can purchase a slide such as this one if your current model doesn't have an 'f' factor conversion chart:
If you've looked at the conversion table, you'll see that the compressibility factor is 1.0 at or below 10,000 feet up to 250 KCAS. The opposite extreme is 550 KCAS @ 50,000' PA-- .84.
So with a CAS of 550 Knots, the Equivalent Airspeed is 550(.84) 462
Now add true air temp and you can find TAS.
Jetguy, cool conversion, I've always used Indicated + (1/2 Flight Level P.A.)=Quick and Dirty TAS (and I mean capital Q/D
) I.E. 250 KIAS @ FL240 = 370 KTAS. And of course this disregards EAS so when I'm flying my F-104 Starfighter X-country I'll have to employ more precision.