The point is that splitting equally is NOT the same as weighing. It is impossible for someone to accurately split evenly a pile of a chemical. Depending on how the individual crystals/powder are laying against whatever is next to it, it can look much smaller or larger than another pile that weighs the same. Eyeballing is dangerous and inaccurate... no one can reasonably expect to be able to do it accurately.
If you want to be sure you have equal doses, use
liquid measurement. It is MUCH more accurate and will improve your safety greatly with these materials. Just take a known amount of distilled water (tap can contain chlorine which can destroy or damage some molecules) and dissolve your chemical into it. If you have 100mg, you could dissolve it into 100mL of water, and then every mL of water would have 1mg of chemical in it. Or you could dissolve it into 10mL of water and every mL would have 10mg in it. Either way, you can measure water volume accurate to within .1mL with an oral syringe you can find for $1 at a local pharmacy. If you plan to keep the solution for a while, it's best to use at least 20% of the volume as alcohol to prevent bacterial or fungal growth (in the 100mL example, you could use 20mL ethanol plus 80mL water, or you could use 50mL 80 proof vodka and 50mL water), but this isn't necessary if you're going to be dosing it all within a short time.
And really, if you're working with chemicals, just buy a scale. Honestly, you can get a decent milligram scale for $30 on Amazon. If you can afford to buy this chemical you can afford to buy a scale. It should be a requirement if you're going to be working with potent chemicals at all.
And it's also good to note that you can't be sure that what you got from a vendor is the weight that they told you. You could have been given less, or you could have been given quite a bit more. I once got, from a reputable vendor, a bag that was supposed to be 250mg, that actually weighed 490. I had a scale so I was just happy about it because I got almost twice what I paid for, but if I had just assumed it was 250, even if I had used liquid measurement, I would have been taking almost twice the dosages I meant to. It is VERY common for vendors to send somewhat different amounts than what they meant to send. If they're reputable, it's usually more, but that can be dangerous if you don't have a scale.
Moral of the story: get a scale! They're cheap to get one that will suit your purposes! And if you really just won't, then at LEAST use liquid measurement, which is free.