One concrete data point for the thread. Two things anyone can check: a state licence number for a 503A, and an FDA outsourcing-facility registration for a 503B. Both are publicly searchable, and a pharmacy unwilling to give you either has answered the question.

One thing that is still open after Dr.KarenChen’s answer:

How much material I am losing to dead space, and whether that explains why a 10mg vial gives me nine usable draws rather than ten?

PeptideSynthNJ said:

Two things anyone can check: a state licence number for a 503A, and an FDA outsourcing-facility registration for a 503B.

There is a second half to this that has not been said yet. They are two different exemptions from the same federal requirements and they buy different things. A 503A pharmacy is regulated primarily by the state board, needs a patient-specific prescription, is exempt from CGMP, and may use a bulk substance that has a USP monograph, is a component of an approved drug, or appears on the 503A bulks list — three independent doorways. A 503B outsourcing facility registers with the FDA, is inspected on a risk basis, must comply with CGMP, may compound for office stock without a patient-specific prescription, and has one doorway to a permitted bulk substance: the 503B bulks list, or the drug shortage list.

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

Closing the loop on my own question.

Resolved. I wrote mg, ml and mg/ml on the vial in marker and did the division in that order, and the two of us now get the same number every time.

sarah_TO said:

They are two different exemptions from the same federal requirements and they buy different things.

Agreed, and it is worth saying that rounding to the nearest whole unit is usually acceptable at maintenance doses and is not acceptable at the bottom of a ladder, where one unit can be a fifth of the intended dose.