Example: FL290 ; OAT -47 degree C ISA standard = 15 – (feet /1000) * 2 ISA Deviation = OAT – ISA standard FL 290 : 29,000 feet, so ISA standard @ FL 290 = 15- (29*2) = -43 degree C ISA deviation = -47 – -(43)  = ISA-4 degree C   https://youtu.be/_oWqu12Dm6M?si=eUYvJoTQD_EOKKl2