I upgraded to dual endstops and seem to have everything working correctly. But when I starting dialing in the offsets, it was struggle. When I entered a m666 x2 for example, I go the same box measurements as when I had m666 x0. I set to extreme to x and y = 4mm but still no change. Then I realized my box was being drawn as a parallelogram so the corner to corner measurement was 1/4" off but the legs were parallel. But m666 commands still did not reduce the error. I then gave up and started moving the end stops around and suddenly I got a corner to corner dead on along with leg lengths. I am assuming this is ok to just adjust end stops until it is square? It was really hard to get a good line to read. I ended up laying down a good sharpie line and measuring to the min/max line thickness. I think that is about as close as I can measure for now.
I didn’t play with the stop offsets at all, and adjusted the stop blocks until I got a nice square result.
I think the official docs suggest this method and just use the M666 for distances under belt a tooth off.