It'll be in the atpl notes somewhete Gengis, probably in a box under the xmas decorations in the attic! Havent read them in years and hope never to have to ever again.....bloody awful syllabus!
The formula is just one of those bizarre things i seem to remember. God knows why. Must've interested me at the time. I just look at the mach indicator or the FMS these days. Funny how we all remember strange things! The human brain is trully amazing.