I suppose my days go back farther than yours. I was riding and working on bikes before Shimano was a serious player in derailleurs.

But, even later after SunTour and Shimano were producing quality road bike components, we were still without much by way of documentation. Even if there were any, there wasn't the internet, so access to them was limited.

I guess you pre-date paper.

Any new Shimano component came with written directions.

No. I don't predate paper.

However, I do predate lots of things people take for granted today.

Have you considered that many of us work on bikes without taking brand new components out of the box?

Also consider that some people are willing and able to use non-textbook gearing. That requires some knowledge beyond the ability to read and follow instructions, which might not be correct for the specific situation anyway.

If things like sizing chains, and working on bikes in general were simply a matter of reading instructions in boxes, we wouldn't need experience or judgement, nor would there be a need for this forum.

Unless you've spec'd cassette and chainrings that exceed the manufacturer's rear derailleur chain wrap capacity (as opposed to max cog size), all you need to do is make the chain just short enough to put a little tension on the pulley cage in the small/small gear. There's nothing wrong with a longish chain - it will shift fine and causes less wear and tear.