We set the rockers with "zero" lash. That means tighten the rocker nut down so there's no up or down play in the pushrod. From there, tighten the nut an additional half-turn. Then secure in place with the Allen-head set screw. This should be done with the piston at top dead center (TDC) so the lifter is on the base-circle part of the cam lobe. The engine then needs to be rotated (either with a socket and breaker bar on the crank pulley or by bumping the starter) each time you move to another cylinder so that each cylinder is at TDC.