I am sure about the quantity of mint (0.015%). Rosmarinic acid (RA) is not very soluble in water and much more soluble in organic solvents such as alcohol. But, I think a water-based eyedrop is safer for the eye than an alcohol-based one, so we should stick with water. As to type of mint, peppermint or spearmint would be OK. Ideally, the mint should be analyzed to measure its RA content. Ideally, the RA content of the final eyedrop should also be determined to make sure we are actually getting some RA. The RA content of the eyedrop can be measured by spectrophotometry, though this would be costly. And last but not least, all eyedrops should be passed through a Millipore filter
(0.2 µm, but preferably 0.1 µm) to remove most bacteria and viruses. UV sterilization is not advised since it might affect the RA in the eyedrop.
The only other ingredient required is an agent to maintain the sterility of the eyedrops, preferably EDTA or benzalkonium chloride (BAC). EDTA is kinder to the cornea than BAC, but it might contain traces of toxic formaldehyde left over from its manufacture. So we prefer BAC, at a concentration of 0.01%.
After preparation, the sterility of all batches of the eyedrop must be confirmed by determining the microbial count to make absolutely sure it is free from all viable organisms.
Alternatively, pure RA could be used to make the eyedrop, e.g., containing 0.015% of RA. If this amount of RA in the drops stings, we could reduce it to a comfortable level at which it does not sting.