To correctly measure out a liquid, you will need to use a Measuring Syringe to provide you with an accurate dose.
These Measuring Syringes are sterile when packaged and will allow you to quickly and easily draw the amount of liquid you require, to add it into your water and they are clearly marked in millilitres to show you exactly how much liquid you have drawn into them.
Dont guess and make mistakes as you may well regret it, use a syringe every time when mixing your solution.