OK, I'll take a stab at this. First off, you're talking about fermenting a lager under pressure, so the title Pressure lagering doesn't quite fit your question.
I've done a few small pressurized lager batches. From what I understand (and someone please correct me if I'm mistaken), it's not the pressure that lets you ferment a lager faster. It's the higher temperatures. The pressure helps suppress the esters that would normally form at higher temperatures.
I haven't tried this yet, but I would fill a 500ml or 1L PET soda bottle with the same wort and yeast as in your pressurized batch, but ferment it without pressure. Use that to check your gravity. When that one's done, your pressurized batch should be done, too. Theoretically. Can anyone confirm this?