Best and easiest way to maintain even hypo is to take a piece of tape and mark the water line on the glass where the salinity is perfect hypo. Just check it often, top off often, and you're good. I do recommend everyone who keeps fish to get a refractometer. They're spendy, but fish are more so over time, and the instrument will last through years. Using that instrument properly calibrated will give you instant, accurate assessment, and that mark on the glass, once established, will be sure you're good. If you want to do a lot of fish-acquiring for a big tank over the years, an inexpensive dual float switch and maxijet pump will keep that salinity bang-on target for as long as you keep the reservoir filled. It's the same rig I use to keep my tank: no fancy controller, just a simple float switch wired to a plugin with a pump.
Water quality does have a great deal to do with your fish health: and having your qt as well as your dt wired with a topoff is one important aspect of keeping that water quality high. I have no way to prove it, but I suspect most complaints of a fish lost in quarantine relates to water quality and the difficulty of maintaining it well: equip yourself for the long haul, and you will find this aspect of the hobby much more push-button and a lot less worry. Evaporation altering salinity and evaporation concentrating a medication are neither one desirable situations: ---and lest you conclude stopping all evaporation could make things safer, evaporation and gas exchange are important to temperature control and water quality, so you can get into trouble not having sufficient evaporation. Best equip the qt to do what it needs to do: hold steady water quality under the difficult circumstance of a smaller volume of water.