I’m building my first mpcnc and got to the point of squaring the trucks after putting the tops on. I understanding that I should move the gantry to some point in the middle, measure the distance, and adjust. Am I reading correctly that adjusting the truck tension bolts will somehow ensure that, if done correctly, will result in the gantry consistently traveling the same distance on both x and both y rails without the belts and core in place? what actually happening here? I have confirmed the frame is square. Does this just ensure pressure is consistent on the rails for each truck?
Just get it as close as you can and the auto square will handle the rest.
Thanks for the response. I have measured my corners many times and am impressed with how incredibly square it is. What I’m not getting is the part at the end of the Tops section about moving the gantry around freehand, measuring, and then adjusting the trolley tension screws to fix. To me this makes no sense. I.e. I did the exercise and found that I was off by maybe 3 mm on one axis, but there’s a bit of play in the system without the belts. Being off 3 mm felt like a random chance thing. I’m sure I’m misinterpreting something or just a bit too dense for this step 
No for some it works and others it doesn’t. I am not sure why. If some are too tight I think others don’t work.
If I make a new one that will not be the adjustment system..