Use a large square against the rail and on top of the wasteboard. There should be no gap between the square and the wide makerslide (as pictured above). If you see a gap, loosen the 4 button head cap screw at the end plate of the wide makerslide, turn the makerslide, and tighten the four screws back up. Repeat on the opposite side as needed. If your carves are still deeper on one side than the other, check the squaring of the Y-axis rails.