OK. Memory lane here
I grew up driving old British machinery made in the '40s & '50s - and your pushrod stories remind me . . . .
Some of those Luton-built Vauxhall/Bedford engines were designed to have the valve clearances set when they were hot and running. They were screw-and-locknut rockers. IIRC there were .007 and .013 valve clearances and Vauxhall/Bedford supplied feeler gauge strips in these two sizes in the form of flexible metal strips maybe a half-inch in width and a couple of feet in length.
The process wasn't complex as there were no "find TDC" issues with the engine running. You simply loosened off the locknut and screw a tad and inserted the end of the feeler strip. As the pushrod moved it would pinch-and-release-pinch, obviously. As you adjusted the screw, you could feel - by pulling the strip back and forwards a bit - when the "pinch" was just right. Tighten the locknut at that and move onto the next . . . .
As you might imagine, a certain technique was required - but not as difficult as it sounds.
However, three hands were needed: One to hold the spanner on the locknut. One to hold the screwdriver that wanted to keep bouncing off the top of the adjuster screw. One to hold the feeler gauge strip. Bear in mind, also, the engine is running sans rocker cover that this point - albeit at a slow idle - so there's black oil everywhere.
The approved technique is: One hand on the screwdriver. one hand on the spanner. Hold the feeler strip between the teeth (also had the benenfit of giving you a good, close, look at what you were doing) and move the head back and forwards a little to "feel the pinch".
When the end of the feeler gauge strip got a bit too battered - just nip it off and move on
Stories of glasses spattered with engine oil are legion. Better, though, is the description of the oil as it migrated up the feeler gauge strip (oil was thin, black shit in those days), how it tasted, and the stains it left in one's beard as it dripped off the edges of the bottom lip . . . .
Those were the days . . .