The closer you get to the salinity of tears, about 9000 ppm I think, the less eye sting you'll get. So 2000 ppm will cause less eye sting than 1500 ppm, and 4000 will cause less than 2000, etc., etc. It's a diminishing returns thing at some point though, so start low and work your way up until it's where it feels good to you.
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