There is no exact way to decide the threshold. To illustrate this, I have taken a profile line to illustrate the intensity of the droplet. When you decide the threshold you are selecting where you are cutting, and as you go up or down you will include more or fewer pixels. One option is to go halfway, or you can go closer to the top or the bottom.
What is very important is that if you are going to do this for a large number of cases, you should be consistent on your decision.
Hope this helps.