It should be calibrated every time you use it with a 32 ppt solution.
Temperature is finicky. That range is kinda old-school as well to be honest.
Nobody really knows the exact origin of that range, the guess is that it was from measuring different coral reef locations.
Coral typically tolerate lower temperatures better than higher ones. I would encourage keeping the tank between 76 and 78 as the max. Above 78 you start to see more stress, bacterial outbreaks, more STN/RTN, and a variety of other problems with fish and inverts.
To be clear, yes, you can get coral and fish, etc. to adapt to the temperature, but that's like playing on "Hard Mode" for absolutely no benefit.
