Hi Johan, thanks for the link to the video (at the exact time). I was aware there was something wrong with my simulation but the results seemed to be pointing against my intuition, apparently confirming that a rotation by 90° was unnecessary. Though, after having seen that point of the video (I've already seen some Robin Renzetti's videos but never went across this one) I started to check again my simulation and found the problem: I used + and - and 0, that caused a mess. Just using colored dots the simulation confirms that 90° is required to detect highs and lows when they are symmetric.

In the case of rectangular plates, though, if the length is several times larger than the width a trick can be used: shifting the plates by one or better two or three times the width over the length allows to change the position of the highs and lows making them detectable.
For example, when I lapped the bars that I used to make the ways of my lathe, I alternatively shifted them by about ten centimeters on each side.

At this point I have to tear down the video I published on the topic because, while I didn't explicitly mentioned this thing of rotating by 90°, I have however shown how I've lapped my bars. That was an unintentional link, but it appears to suggest otherwise.

Cheers, Claudio.