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)