Pick the water volume before you pick the syringe
Reconstitution is one division, and the whole thing goes wrong at the syringe rather than in the arithmetic. A lyophilised vial holds a stated mass. You add a chosen volume of bacteriostatic water. What you get is a concentration:
concentration (mg/mL) = vial strength (mg) / water added (mL)
Then the draw:
volume to draw (mL) = dose (mg) / concentration (mg/mL)
Worked example
A 5 mg vial with 2 mL of bacteriostatic water is 2.5 mg/mL. A 0.25 mg dose is 0.25 / 2.5 = 0.1 mL. On a U-100 insulin syringe that is the 10 unit mark.
Reading a U-100 syringe
U-100 syringes are marked in insulin units, not millilitres. The conversion never changes: 100 units = 1 mL, so 1 unit = 0.01 mL. That means 0.05 mL is 5 units, 0.10 mL is 10 units, 0.20 mL is 20 units, 0.50 mL is 50 units.
The part people skip
Choose the reconstitution volume so that your ordinary dose lands on a mark you can actually read. A dose that works out to 2.3 units is a dose you will measure badly every single time. If the division keeps producing awkward fractions of a unit, add more water and redo it. More water is not weaker medicine, it is the same mass spread over a volume your eyes can resolve. The trade-off is only how long the mixed vial lasts and whether the vial has room.
Notice that a 5 mg vial in 2 mL and a 10 mg vial in 4 mL both give 2.5 mg/mL and behave identically at the syringe. Concentration, not vial size, is what the syringe marks answer to. I run the division and the unit conversion through the Peptide Tracker & Calculator rather than doing it in my head at 7 am, because the failure mode is a silent factor-of-ten error, not an obviously wrong number.