The dosage and the time

When I was using antibiotic eyesdrops recently, I was a bit concerned: the precise dosage is supposed to be important with antibiotics, but how do you achieve precise dosage with eyedrops? How do you control the drop size? How can you control or even estimate how much of the drop you just put in your eye flows out onto your cheek?

The hair color instructions always make me wonder as well: they always say things like "put the stuff in your hair, wait 20 minutes, wash it out". However it takes more than 20 minutes just to put the stuff all over your hair, and by the time you are done the part where you started has already been covered with dye for longer than that. Are you supposed to start washing it out slowly, beginning with the parts where you put the color first? Are you supposed to color your hair a bit at a time, so that this is not an issue?

Nowadays I just go to a hairdresser, and she puts the color on and then waits 20 minutes after she is done, and I have never noticed any color difference between the parts where the color has been for 20 minutes and the parts where it has been for 40 minutes, but it's hard to imagine that this wouldn't be an issue with some colors.