Ahh... the road of water which ends in paint (pain?) if not done correctly.

Bottom line... Sometimes your mistakes will cost some coin, but really... for a few drops of paint that you may have screwed up on?? Bite the bullet and start over (unless you used the whole pot?) You will learn in time, like the others have said, and it is a feel thing. You will get the hang of it shortly. Practice makes perfect.

But all that aside, I use only one brush for all my mixing and thinning etc. With just this one brush, I have learned how much to dip for how much water I need to thin what paint I have out; how much to glob onto it before I mix, add anything fancy or what not. I contstantly clean this brush and since I always use the same brush, I have learned a benchmark to judge my proportions with. I never paint with it, only mix and proportion with it. Just be careful you don't contaminate your other paints when mixing!