One way to determine if the locking nut is set to give a good buzz-free 1st-fret action is to get a 300mm steel ruler and put it on the fretboard so that the edge is touching the frets, then slide it so that the end is touching the front edge of the nut, if the nut is set to the correct height you will see that the bottom of each string-groove on the front of the nut is just slightly above the edge of the ruler that's touching the top of the frets, if not, you need to add a thin shim underneath the locking nut to bring it's height up a bit.