6 sounds very close, I wish I'd measured the slot more closely when I had the cover off my M42. But the tooth to tooth spacing was about even with the length of the adjustment slot.
I degree cams similar way, use the valve not the cam lobe. 2 pin dial indicators one for the piston one for the valve. Haven't done the M42, usually working on a Cosworth Twin cam engine in a midget. We have a hand cut front pulley that matches the crank to the cam belt pulley (Corrected for about 1.5 degrees error in the keyway).
One of the downsides of a belt driven cam set is the belt stretch at 11000 rpm's...
Dave