you have to have the locknut loose and keep a box-end 12mm over it so it wont tighten when you turn the screw. then use the feelers with one hand and adjust the screw with a screwdriver. when you feel some drag but can still move the feeler, hold the screw with the screwdriver while you tighten the locknut with the wrench. the goal is to keep the screw in the position you found, it will try to change when you tighten the nut. after you tighten it, recheck with the feelers. if its still good, tighten the nut to spec, then check with the feelers again. if you dont do a lot of rechecking, you will have wasted your time.