Okay so now that your frame is square the belt tension is what squares the X rail.
Pull on one of the belts going across the back, watch the sides of the X rail, one will move towards the back.
To set the X rail I move it all the way to the corner blocks and you can see how crooked it is. Loosen the linear rails 4 screws a bit so it can move and tighten and loosen the AB belts until it is straight. Make sure the A block is always forth out front so it can trigger the switch before it runs into the B block when homing.
Snug up the rail and then you can dial this in to any degree of perfection you need with a real print. I print the temporary LR3 struts and measure the diagonals of the print. Then make tiny 1/4 tension adjustments to the belt.