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.