[Amps] 4CX1500B

[Peter Chadwick]

>With 0db attenuation, take the S meter reading at the adjacent 
frequency, tune to the fundamental frequency, add attenuation as needed to give the same S-meter reading as before. The # of db of attenuation 
is how many db down the total IMD is.<

Even this depends on how accurately you can set the S meter to the same place. If we have a meter range to S9 + 40, and even 4 dB per S point, that's a 76dB range. So to set to 1dB, you have to get the meter to within around 1.5% of the same value - and on a speech signal, too.
73

Peter W6/G3RZP