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
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.
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
So the question, as narrowly as I can put it: 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. Not looking for reassurance. Looking for the part I have got wrong.
— sarah_TO · corrections welcome and will be edited into this post with credit