[Jim A]

I am afraid I have never understood the purpose and process of adjusting valve lash (clearance) while an engine is running.
First, I think damage to the gauge is possible through hammering.
Second, with the gap opening and closing around 300 times a minute at idle, how do you "feel" the exact resistance you are looking for?
Third, it is often hard to tighten the locknut without changing the gap when the rocker is still and not bouncing up and down those 300 times.

To overcome these difficulties, some real advantage must be gained by the running vs, not difference.

I should say that I have adjusted the valves of probably over 100 engines of over twenty makes over a period of 60 years.