Sorry Clive,
When I read you post I thought that you were saying "Multiply the pressure altitude by the mach number and you get the EAS at that mach number".
For one fleeting moment I thought that this was going to be one of those really magical moments like the day I was first introduced to Calulus all those decades ago.
Now I see that what you were really saying was "If you have a table of pressure altitudes against the EAS for local speed of sound, then just multiply this EAS by any given mach number and you will get the EAS for that mach number at that pressure altitude".