Dr.PathRoch said:

Practical: whatever you change, write down the date and the reason.

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.

Dr.PathRoch said:

Practical: whatever you change, write down the date and the reason.

That is right, and it stops being right at the edges. The general case is well behaved; the interesting cases in this thread are all at the boundary where the general case breaks.

Dr.PathRoch said:

Practical: whatever you change, write down the date and the reason.

Thank you — that is the clearest version of this I have read, and I have read a lot of them.

From the other side of the consultation, briefly. The version of this that has an answer is narrower than the version being asked. Narrow it and it becomes tractable; leave it broad and the thread will produce nine confident and incompatible replies.

Closing the loop on my own question.

Following this thread I went back and re-read the extension data properly. The point about trough rather than peak explains the pattern I was seeing on day six, which I had been blaming on the vial.