I think you're still missing the fundamental point that your examples have nothing to do with the volumetric aspect of the bac water. Your statements are just about dividing the mass of the vial differently into doses: 30= 10 x 3, or 30 x 1, or 15 x 2
You need to understand the relationship between the two volumetric measurements milliliters and units, and then understand that you're just diluting the mg contents of the vial to hit the target mg in your selected number of units.
...which for me would ideally be anything between 10 & 40 units as number-marked on my chosen syringes. Especially if you use the despicable 100 unit ones. And if you have 30 unit syringes, you definitely don't want to pin 40 units each time.