In my experience sweetbreath vials stay completely sealed to prevent evaporation over long periods of time, but they only hold 3.7ml. As would many different scientific vials with greater capacity available for purchase.
20mgs per ml sounds high. Much too high to be using a graduated cylinder. Pipets, although accurate, are a little difficult not to spill, and spilling expensive chemical solutions is not fun.
If a person could be sure they have prevented evaporation, a .5ml syringe would provide the precision to dose with confidence a 10mg per ml solution. Even if you were off by a full tenth of the syringe, it would only be .5 of a mg. In my experience, I'd estimate my accuracy to within +-.2mg when using this method.
Of course, using a larger volume of water and a small syringe would be even more accurate, although slightly tedious.
If the method of measurement is brought in line with the concentration, AND evaperation has been prevented I don't believe there is much added danger (as long as the chemical does not fall out of solution).
If you don't have good reason to make your concentration high than make it low like the standard 1mg per ml. I use a 3ml horse needle and a small mason jar at that concentration. I had a good reason to use a high concentration, otherwise I use 1mg/ml.