Your method would work but I would suggest verifying the meter behavior first on a known waveform before trusting it
DC mode on some might average linearly but you would need to experiment on known waveforms beforehand as it’s not guaranteed what a meter will do with a pulse train when set to DC mode.
AC with true RMS is a guaranteed way for this method to work everytime but ya just got to calculate the expected rms first.
-s
