i am no authority, and if i was not feeling so talkative, and over zealous i would not have tended to this question,
because, again i am no athority.........
but I am an egger unseasoned bee trying to fly straight
looking for quality answers, hoping for quality results,
i assume like you
i have had experience when it comes to scales,
buy them, you need them, it is crazy to approximate weight with you eyes, there are so many variables that make it unfeasible.........
reflection from the bag, humidity, etc, etc, i am sure
seasoned bees could give you even more.
you can get hand, and digitals from you local head shop, the digitals range from .01 up usually to 112gms (have gotten them in the 100-$200 rang), at head shops where i'm flyn' & even more cost efficient, less accurate scale that start at 1gm.
and if you are feeling extra exocentric :-) and through you can go to a lab ware supplier for super good accuracy,
but no matter what,
scales are a necessary, whither you have intentions of synthesizing meth or not.
In my humble, “stranger” titled opinion.
good luck