Singer 66 Series Service Manual page 14

The feed dog should be set so that when it is raised to its highest point by
the feed raising bar, the top of the feed points will be about 3/64 inch above
the top surface of the throat plate.
When it is necessary to raise or lower the feed dog, loosen the feed dog screw
(N, Fig. 8) and set the feed dog in the required position, then securely tighten
the feed dog screw (N).
To Raise and Lower the Feed Dog