Writing this once so I can stop repeating it across threads. It is about the dosing arithmetic, and it is deliberately narrow — everything I am not confident about is marked as such.
What is actually established
Concentration choice is a precision decision, not a preference. Reconstitute high and every dose is a tiny volume where one unit of syringe error is a large fraction of the dose. Reconstitute low and you get more graduations per dose, so the same hand tremor costs proportionally less. Against that, more diluent means more benzyl alcohol and a shorter comfortable in-use window.
The condition it depends on
The arithmetic is right and one caveat matters: a U-40 syringe changes the markings, not the volumes. If someone hands you U-40 and you read it as U-100 you will be 2.5 times out, and that error has a direction — it is always an overdose.
The practical version
The three numbers to write on the vial: total mg, total ml, and mg per ml. Everything else is division. And the sanity check is that dose volume times number of doses should be less than the volume you put in, because dead space takes the difference.
What I am not sure about
What I am trying to establish is how to check the arithmetic without trusting a website calculator, since three calculators gave me three answers. If the honest answer is that nobody knows, that is a useful answer and I would rather have it.
— emily_PDX · corrections welcome and will be edited into this post with credit