Measuring sine amplitude with ADC
If you can be sure that your passive network is perfectly linear (does not create harmonics) you can just use the raw min/max values that you read from the ADC and compare them (correctly scaled) to the min/max values that you put into the DAC.
Some inaccuries will arise for sure, ie. because the DAC and ADC are not perfectly linear. If your passive network has low impedance, you would also have to take into account the internal resistance of the DAC.
If you need high precision, doing a FFT or Goertzel is the way to go.