I use a 12" carpenter's "torpedo" level.
Ya gotta level your platter to the best of your ability. When mine was off about 1/3 bubble, I was getting the weirdest phase anomalies and balance problems. On one mono album, the sound seemed to come from from the front window to my left--90 degrees left of the point between the speakers.
Using my trusty Sears Craftsman level to get the bubbles to level longitudinally, laterally, and diagonally fixed all this.