I don't know that anyone has really tested the the P12 OEM car to see where exactly it engages (mine is a 20V in an AUDM N15), but that's where the low cam starts to taper off so generally, doing a run with the low cam past its point of usefulness then doing a run with the high cam from well below its normal point will see a 'crossover' point which is pretty much where you should switch cams, give or take a couple a hundred rpm. That's where it works for my car with my setup, which is basically a stock standard setup.