Once you're set up and running for a while, you should be fine. You'll have it all planned down to a science on how many scoops (1/2 cups) of salt for the bucket you mix your sw in for your water changes. Eventually you won't see sg levels fluctuate like that anymore. Also, the bigger the tank, the more stable your parameters will become.
I have been getting great (accurate) readings with my Deep Six hydrometer. I actually bought a refractometer a couple weeks ago, but I've been to distracted with my new powerheads to check if it's calibrated right before I trust using it.
We should be seeing rain all weekend, so I'll get around to it then. Good luck in the future with Krusty. Sorry to hear about your wrasse though. Sad, but better to learn this stuff now than later on a fully stocked tank.