Do not eyeball anything, except in rare cases some RC's are forgiving, like AMT or 2c-I for example. If you have a normal cheap scale (not electronic), you can weight 100mg and split that in two and get more or less two piles of 50mg, or in 4 and have more or less 20mg of 2c-I. I never had problems with such, but I had 2c-E at one time and didn't do it that way, as I heard 2c-E is NOT forgiving, as like 20mg can be comfortable for one person, but 22mg will be a trip to hell for the same person...
With anything requiring smaller doses get an electronic scale. I only have an electronic scale that goes 2 numbers after the "," that should be okay for some product, but definitely not okay for others. I wish I had a 0.000 scale but they are much more expensive. This 0.00 scale cost me 74 dollars. It's worth getting a milligram scale if you are playing with things that can be easily deadly (not chemicals I mentioned....most tryptamines in general).
A friend of mine once messed up weighing his 2c-I using a regular non electronic scale and he had much more than 20mg like he aimed for, probably had 60mg and since then he has heart problems of the electrical kind, he could go into ventricular fibrillation anytime if he forgets taking his meds. His life is ruined partly, because he cannot do any drug or even drink alcohol now, since 6 years, did that when he was 20. Just a warning.