The value you need to set the slider to depends on the font you use and (maybe.. I thought it didnt) on the pixel size. You just slide it until the distance between syllables is the same as the distance between letters in a syllable
And its basically the reference value for the tracking amount of spaces. You could say that a certain value means 0px width. If you go lower its a negative value, so the syllables get closer together. If its bigger than the desired value, the spaces added for making syllables 'words' (AE recognizes them as such when there are spaces between), can still be seen and the syllables will look like real words (with spaces around them) instead of syllables of a word.