JenPlateau said:

It helps to say which part of this you are uncertain about.

Second this. Nothing to add that would improve it.

JenPlateau said:

It helps to say which part of this you are uncertain about.

Adding the part of the answer the thread has not reached. 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 correction. That is slower than asserting, and it is the only version that survives being wrong.

One concrete data point for the thread. One practical note: write down what you did and when, before you need it. Reconstructing a timeline from memory three months later is how people end up unable to answer the one question that would have resolved it.

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

Did your prescriber agree with that reading, and if not what was their objection?

Closing the loop on my own question.

Update: the thing I was blaming turned out not to be the cause. Leaving the original post as written rather than editing it, so the mistake stays visible for whoever searches this next.