LibrarianMeg said:

Whatever the answer turns out to be, the method for getting there is the same: state the assumption, do the arithmetic in public, and invite the…

Adding the part of the answer the thread has not reached. Take it one variable at a time. Almost every unanswerable question in these threads is unanswerable because three things changed in the same fortnight, and no amount of subsequent argument can untangle them after the fact.

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.

That is the short version; the long version is somebody else's post.

Following on from LibrarianMeg — and this may be the naive question:

What would you measure differently if you were starting again?

Dr.PathRoch said:

For anyone reading later: the numbers in this thread are worth checking against a primary source before you act on them, including mine.

This is where I part company with the consensus forming above. The counter-case has not been addressed. Somebody upthread described the situation that does not fit, and the thread moved on rather than engaging with it, which is the failure mode this board is supposed to avoid.

Closing the loop on my own question.

Closing this out: the trough rather than the peak explains the pattern I was seeing on day six, which I had been blaming on the vial.