My 312 has the flat screw head and 5/8 locknut on the rocker arm. Trying to hold the screw steady while tightening the locknut is a real hassle as the screw always wants to turn just enough to throw the setting off. I wind up setting the lash wider trying to compensate for that little bit as I tighten the locknut. I saw real good video of a fellow adjusting valves on a Y-Block using just a feeler gauge and a socket style wrench, each valve took him a few seconds to set the lash. Does anyone know of this type of setup?

1955 312 T-Bird Warwick, NY