fogie: the normal position is in the middle. If for a string the high frets sound too low in relation to the empty string, move its slider in the direction of the fretboard. In the opposite case move them to the other direction. If some frets come too low and others too high, move the whole instrument back to the dealer - well, try a new string first.

Why? Fret positions roughly follow school mathematics, but only roughly so. The current mechanical condition of the string is important, but the player's individual technique has an impact as well (which may sound surprising but is evident from the fact that the left hand can produce a vibrato).