[orcafood]

I like to have my local fish stores and fellow reefers test the water of the fish or frags I buy, then take a note of the deviation. After a few different stores and fellow reefers my refractometer is nicely calibrated, who doesn't constantly buy frags? They will likely even test the water for you right there to be sure.

I definitely advise against calibrating with ro/di. No way to guess slope and/or offset error amounts.