I had a prop valve that leaked just like yours. I never found any way to repair it. I ended up replacing the unit. Problem solved !!
I always mark the unit and the knob at the closed (left) and open (right) position . I then mark at the adjusted position. It helps me remember in case I ever need to change the adjustment.
Right or wrong, I count the number of turns from the closed position (left) to the open position (right). Turn the knob slowly and softly.
I then turn to the closed (left) position and turn to the right 60%. That has always been a good starting point for me. Not all cars will be the same, so adjust it as you need to and good luck.
I would like to hear any other comments about this. Bring them on guys !!
I hope this helps.