Well I had a friend who is a lab tech { hobbyist} for a sate lab to show me yrs ago that If I checked S/g using a refractometer I should allow all fluids to get to a room tempature 70 degs F and then use some distilled water to adjust to zero then use Witch Hazel and to double check the set/calibration of the unit . Using the witch hazle last to double check the distilled water the unit should read 43 ppt then that will or would equal at 1.030 that. Its fast simple and very accurate. And uses all common household products setting most peoples medicine cabinets . Well the witch Hazel anyways :>) the distilled water is another matter.
But all the fluids used to be tested for the calibration should be the same temp as the refractometer. And who's to say the room ambient temp and the fluids temp or the meters are perfectly adjusted in temperature ? It could cause slight distortion in the readings for calibration . Close is good I would think. I doubt a ocean has a constant S/P gravity reading 24/7 365 , In-fact as most tropical reefs are closer to land then I'd assume that rain run off would cause more fluctuation around them then from further out in the open ocean ? But that just me using common sense again got no data.. But I'm sure someone does
Also uing this methoed last time I checked a refractometer I like to run mine near 30 ppt or 1.024 1.025 + or - a little