100 mg is the most I'll ever use, and the most anyone should use unless you're a chemist.
I would agree with this, or at least extremely skilled with 100 mg/mL and have all the safety requirements, and use them. 200 mg/mL is more than I am comfortable with, although I have worked with 300 mg/mL. You will need good ventilation. The fumes can give you a serious buzz. Bad on the skin, gloves and long sleeves are a must. But even nitrile gloves can allow some absorption if enough gets on them. And yes, it will sting the eyes, but also realize that the eyes will absorb the nic quite effectively, sending it straight to the brain. So safety glasses are a must. And if you spill a bunch, especially on a carpet, you may need a professional hazmat crew to clean it up (they will probably have to burn the carpet).
Please do not be a Darwin award winning detriment to vaping! An injury or environmental disaster would be a field day for the media and the FDA, and a hospital stay (or worse) would be filled with ridicule from the
vaping community. I personally wish vendors would only sell >100 mg/mL nic to other professionals well-trained in handling very high concentration nic, either
e-liquid manufacturers, chemists or others that can prove they have the experience and are taking safety precautions. I think selling 200+ mg/mL to regular consumers is a disaster waiting to happen.
RTS sells even higher concentration, but in the end I think the price is not worth it, since you cannot get free shipping with it, and shipping is more expensive than usual. And the price of the total nic you get is not much cheaper than the same amount of nic at 100 mg/mL. For me the space savings are not really worth the extra cost and safety concerns. And in the end, I would not want to mix with 200 mg/mL and have it sitting around, and would have to mix it down to 100 mg/mL for usage anyway.