James, it's no so much a meter of the depth of the screw, but ratherthe screw is simple the pivot and anchor by which the lid sits, either farther back, middle or maybe up front all depending of how the screw holds the lid in a certain position according to how the callmaker chooses to tune the call. The depth of the screw really doesn't matter from a technical standpoint, but setting it deeper naturally posityions the lid farther up front and the more you back it out the lid will sit toward the back.
Many callmakers tend to tune their calls toward the back, some much closer to the apex of the rails...it all depends on the sound their are trying to achieve/desire. There is no magic really or a set angle, but mmore a feel, or the knowledge from having done it so many times that each callmaker inherently knows where he wants it to rest as he tunes.
Hope I helped...looking back my answer was much longer than it need to be.