Perhaps the screw is the positional indictor. The numbers on the creme colored cover sure do look like adjustment numbers.
If you feel adventurous, mark the current position, loosen the two nuts a little, turn the cap on way ot the other and RETIGHTEN the nuts. See what effect it has on the wiper function. If it is not right, try the other way. Return it to the original position if it does not work at all.