Rivet Gun
So in EAS terms there was a about a 21% difference in the position of the yellow band between FL80 and FL 380.
In CAS terms the difference was about 27%
Like you I refreshed my memory on the magnitude of the effect. In my case I took the data from my 'bible' - Dick Shevell's Fundamentals of Flight.
For subsonic aircraft the differences aren't too large - at 250 kts CAS/FL350 for example the difference is 12 kts, but if you have a supersonic aircraft and high altitude it becomes more important - at M 2.0/FL500 it is about 80 kts. If anyone is sufficiently interested Shevell gives his source as USAF Series T.O. 1F-5a-1 Flight Manual