A decent "WAG" to figure out kts IAS from TAS is TAS - (FLXXX / 2). Ex: 450 - (FL300 / 2 = 150) = 300 IAS. Going the other way, kts TAS = (FLXXX / 2) + IAS. Ex: (FL300 / 2 = 150) + 300 = 450 kts TAS. It's pretty darn accurate unless you've got some extreme ISA deviations. .76 at FL300 is actually about 447 kts TAS and 287 kts CAS at ISA.
Last edited by TankerDriver; 02-05-2016 at 01:01 PM.