andrew_nyc said:

Order of operations matters more than any single choice here: establish a baseline, change one thing, wait long enough for it to express itself, then…

Adding a me-too, because a thread of one person's experience is not much use.

andrew_nyc said:

Order of operations matters more than any single choice here: establish a baseline, change one thing, wait long enough for it to express itself, then…

Coming at andrew_nyc’s question from a different direction. The mechanism and the magnitude are separate questions. Agreeing that something happens says nothing about whether it happens enough to act on.

If somebody has the primary source to hand I would rather cite it than paraphrase it.

The figures, for anyone assembling their own picture. For anyone reading later: the numbers in this thread are worth checking against a primary source before you act on them, including mine. Half the figures circulating in this community trace back to a secondary summary that dropped a qualifier.

A narrower follow-up, since the general answer is now clear:

What would you measure differently if you were starting again?

Reporting back.

For the archive: it took about six weeks rather than the two I expected, and the difference between those two numbers was the whole of my problem.