@W.g Peter’s video is excellent, I found, but we cannot adjust the Mill the way he adjusted his CNC. You have two choices, as I see it. Others may well have others. You can remove all the screws from the Y gantry, tap it back, clamp it and make your test again. When you have it spot on, replace the screws. The issue with that is that, it is likely that you won’t move the Y gantry enough to be able to drill new holes for the hold downs. That was the case for mine, anyway. The other solution is something of a “cheat”, and is the one that I chose for my Mill. My Mill was out of square by about 1/16". I put a 1/6" shim at the front of the Y gantry, brought the Mill all the way to the front until the X gantry hit the stops. Then I ran Peter’s test again. It took me 3 or 4 times to get the thickness of the shim spot on. If you choose to fix yours this way, you need to remember that you cannot choose to “square” things up by running the Mill to the back. (I guess that you put a shim of equal thickness at the back corner opposite the one in the front corner. I didn’t bother.) You will note in Peter’s video that, when he had his dialed in, he ran his X gantry close to the front limits, then moved is limit stops to touch the gantry. That’s basically what I did with the shim. So far, I’ve tested mine a few times in the months since I did this, and it has stayed spot on.