Hi,
Maybe a dumb question: If you know it's 400Vpp ... why do you want to measure it?
Do you expect 50% duty cycle?
Think about this:
400Vpp square wave, assumed 50% duty cycle, can be:
* -200V to +200V --> 200V RMS, 0V average, 200V full wave rectified average
* 0V to 400V --> 282V RMS, 200V average, 200V full wave rectified average
* 1000V to 1400V --> 1217V RMS, 1200V average, 1200V rectified average
So if you really want to measure Vpp then you should do so. Measure maximum, measure minimum and subtract both values.
Klaus