Before tightening, make sure that the trackpad is freely moving up and down. If you feel stiffness, try to very gently adjust it so that in its untightened position the lower part easily moves up and down.

You can tell when the screw is tightened just right by the noise it makes when the trackpad clicks. If the set screw is too loose, the trackpad will have excessive play before it clicks. If it is too tight, the trackpad will click too easily and won't make the characteristic loud mouse clicking noise.