I don't have to calibrate mine often. In fact the calibration screw is rusted over a couple of years ago and won't move but it holds steady in it's readings and cross checks consistently with other devices and refractometers. When I did calibrate it used the 53ms sloution. It's best to calibrate closest on the scale to the typical value you will be measuring.. This minimizes any small inaccuracy in the instrument which will maximize the further away on the scale you calibrate. I cross check the refractometer with a salinity monitor and just got a digital read out refractometer which is neat and easy on the eyes. Redundant measures are useful for my peace of mind.
Top of to a fixed level in teh sump will keep it realtively constant but there is still salt creep and salt mist to account for.
I do not prsonally use a float switch ato so I can't comment on them, I've always worried about stuck on float switches. I know there are ways around it( dual switches etc) but never found it necessary.. I know how much evaporation will occur daily after measuring it for a while so I use a peristaltic pump from a still reservoir full of limewater . This doses my calcium and alk along with top off . The pump is set to deliver 18 liters of limewater per day to the 600 g system and breaks that into 150 equally spaced and sized increments over 24 hours. I do adjust the total amount by a liter or 2 seasonally.
I think sg is a very criticial parameter for inverts including corals since they are isotonic. That is, their internal sg varies to match the water around them ,as water diffuses in and out of them. . They have some variable ability to adjust their internal sg chemically but not much. They can't control it like fish can by osmoregulating, ie drinking and urinating and renal activity When the internal fluid levels go off kilter it messes up the internal chemistry leading to a loss of homeostasis and death, just like it would for us if we became severely dehydrated or over hydrated.
