Lister CD cam arm clearance

Hi every one ,does anyone know how to get the correct clearance [.5 - .75 mm ] between the camshaft operated rocker arm cam follower and the underside [bucket bottom] of the injector pump on a Lister CD,with the pump bolted down it seems as though there is a constant pressure on the underside of the pump by the arm, there is an adjustment screw on the end of the arm, but I am reluctant to alter this setting,I understand this clearance is to compensate for any manufacturing discrepancies ,Any knowledge or information would be much appreciated . Thank you in anticipation. Michael.