Determine your measurements. We will be using a 99.1% water to 0.09% benzyl alcohol solution. For 10 ml of bacteriostatic water, you will need 9 units of benzyl alcohol (0.09 ml) and 10 ml of sterile water (or 9.91 to be exact).
Measure out the benzyl alcohol and put it in the vial that will hold your final solution
Measure out the sterile water and put it in the vial with the alcohol.
To make 30 mL of bacteriostatic water, you need 27 units of benzyl alcohol (0.27 mL).
To make 100 mL of bacteriostatic water, you need 90 units of benzyl alcohol (0.9 mL).