Hello Brian. To tune the call that you are specifically talking about... in terms that you will understand you have two easy options. First, sand by hand the inside and upper areas of the rails to make them thinner. Sand for a couple of minutes, then play both sides, then sand, then play it, repeat.... Or, change the profile of the rails. The profile being that gentle curve. Change it a bit to make the call play farther away from the screw. By playing I mean, look at where the paddle contacts the rails, move that point a bit farther away from the screw by changing the profile "curve" of the rails.
"Instruments of Mass Destruction"