Trial fit of a plate for the magnetic base of the dial-gauge, secured to the exposed threads of the two end cylinder-head studs. The plug leads have been moved out of the way, the plugs themselves will also be removed to make turning the engine easier.
Positioned with the dial-gauge plunger resting on the valve-end of a rocker.
The measurements will be taken with the rocker fully up, so the fact the top of that end of the rocker isn't flat and smooth makes no difference, as the dial-gauge won't be moved between measurements.
The strict rule-of-nine point. The actual reading is immaterial, it is the comparison between this rotational position of the engine and cam with another that is important.
A few degrees one side of the strict rule-of-nine point, and 0.035mm or 1.4 thou greater clearance.
Out of interest a comparison with the valve end of the rocker pushed fully down to indicate the clearance: 0.4mm or 15.7 thou.