You'll know its adjusted properly when you press stop and the platter comes to a stop with little to no additional movement. If you don't have a feeler gauge, take a piece of copier paper and cut it the width of the servo head. Fold the paper over once and use the folded piece of paper as the feeler gauge.
The feeler gauge should fit between the servo head and the platter and you should be able to remove it with a slight friction caused by the head and platter holding the paper. Don't adjust the head so the paper is difficult to remove. It should come out easily but you should feel a slight resistance.
If this does not fix the problem, remove the platter and inspect the inside of the platter where the servo head reads the platter. There should be a coating which is essentially a magnetic coating which holds recorded pulses that the servo head reads. This coating should not be damaged.
The servo can overcome small vertical scratches but any larger gaps will cause the turntable to not be able to lock into the speed pulses. The only way to fix a damaged platter is to find a replacement platter.