Two different “units” questions
There are two separate questions that both get called “units”: how much bacteriostatic water to add (usually measured in mL, not units), and how many units to draw of the final mixed solution for a dose (measured in units on an insulin syringe). Mixing these two up is a common source of confusion.
Measuring the water itself
The water added during reconstitution is measured with a standard mL-marked syringe, often 1mL or 3mL, not the insulin syringe used later. Getting this volume right is what determines the concentration everything else is based on.
Reading an insulin syringe correctly
A standard insulin syringe holds 100 units per 1mL. Each unit therefore represents 0.01mL. Reading between the printed unit markings (most syringes mark every 2 units) accurately, at eye level, matters more than people often expect for small doses.
Common misreads
Parallax error, viewing the syringe at an angle rather than straight on, is one of the more common sources of a slightly wrong draw. Air bubbles in the syringe barrel are another, since they take up space that looks like liquid volume but isn’t.
Getting it right
Confirm the concentration first (from the actual water volume used), calculate the target units from that concentration, then read the syringe at eye level with any air bubbles removed before injecting.