At the mower shop I worked at we never used a tach on trimmers, there are too many variables. Ultimately what will govern the speed is the length of the line. To tune, you want maximum line the guard will allow, flat out, wind the screw in until it leans and revs go way up, screw it out until it 4 strokes (labours -rich) and screw it back in until you find the sweet spot where the note is just on the smooth side of rich.
