I've also started measuring differently, still from the top with the ruler but instead of de wide side of the ruler parallel with the shaft, I turn it 90 degrees so that it's perpendicular. The length that comes out of that it around 0,5cm shorter than the other way. But it feels more accurate, because if I turn the wide side parallel with the shaft it goes deeper into the base and I think it may even go a bit under the public bone. So, for me this way works better. It now goes to around 18,5cm which is where I started... which is fine. I'll work towards 19cm BPEL this month!