A few things as I have the machine and have made a good series of mistakes when squaring it up.
- I would not trust any dimensions posted, based on what I have seen from how these are constructed. I think they could easily be off by a millimeter or two.
- I would not rely on stall homing (“maxed out” to go to the back corners. It isn’t consistent against the back (or to the right) and I have found manually running it to the rear (or side) limits will cause cause the offset to be off.
I think the best approach is to follow the @GeeksWoodShop general guidelines just use some algebra to figure out out your own measurements. That will likely get you to within a few millimeters or so. Than use the method I described in my post linked below, to draw a large square and check the difference between diagonals and if needed shim. While I have found my extra long metric ruler very useful as other have mentioned you don’t need it. All you need to do is measure the delta between the two which could be done with a piece of aluminum or angle.
Just remember to rehome after each adjustment and make sure it is actually making contact.