Joe, our good friend Jeff Evans put together a really good video describing this very thing. Monitoring the shaft plane and sweet spot plane using a devise like Whip's.
Kevin
Thanks Kevin. But here's the problem. In the video, Jeff is rotating the sweetspot around the shaft until it finally lies on the plane board with the shaft. At that point, the sweetspot hasn't shifted planes, but rather its plane gets deformed, making it 3 dimensional. But here's the bigger problem: The job of pp#3 is to feel the sweetspot and if any part of the shaft between pp#3 and the sweetspot touches anything, you will sense that contact point, rather than the sweetspot. The shaft must only touch AIR.