I'm working on an old Singer sewing machine. How should I adjust the (highlighted purple) screw in the image below? It sets the depth that the needle goes down below the clothing being sewn. If I set the nut where it has been locked for decades (based on corrosion and scratch marks) the machinery around the bobbin hits the needle and pushes it forward, against the direction the fabric travels while being sewn.
I'm looking for a spec along the lines of, "The needle eye (or tip) should go 1/8 of an inch below the bottom of the sewing machine foot, when the foot is in the down position..."