If it ain't broken, don't fix it.
If you measure out 20 ml of water, and get 19.87 grams, you can assume linearity of at least. 1%. This assumes you get 9.9 grams when you measure out 10 ml of water. So what is 1% error in 10-80 mg of psychoactive compound?
BTW, as osmium already pointed out, you can't use water (or any other liquid) to calibrate your balance. Getting an exact volume out, right down to the microliter is *hard*. Plus, if you weigh it, you'll see your calibration altering continously because of evaporation and all that stuff.
You almost sound like you're desperate to tweak. Please don't. Chances are, the thing is as good as it is, and you won't make it better.