I used a digital caliper to measure the height from the bottom plate to each leadscrew. Left the pulley screws loose until all 4 corners were at the same height, and then tightened all 4 corner pulleys. Best to do this once the top cover is in place.
Then slightly loosen the screws holding the members to the leadscrews, and electrically (i.e. using the motor controls in LaserWeb) lower it all the way to the bottom stop, tighten the screws and try raising it all the way to the top stop. It should work without the belt skipping over teeth!