Comments inserted into Dennis's text.
If there is enough interest I can show the full calculation so you can do it yourself at altitude. I can also expand the interesting part of the chart and put in some additional diagonal lines to allow one to resolve speed better. Remember: with log scales, the line for 275 mph is NOT halff way between the 250 and 300 lines.
If enough are interested, I will write a little monograph for the archives that has the data on atmospheric density, simple calculation methods, conversion factors, and all that jazz so you can do it yourself.