[Ninong]

If your hydrometer is calibrated at 77 degrees Fahrenheit and your water temperature is 79 degrees Fahrenheit, that's close enough for government work. I wouldn't worry about the slight adjustment.

However, your specific gravity of 1.021 is really low. You should raise it to 1.025 - 1.026. You can do this gradually by using freshly mixed saltwater to replace evaporation until your specific gravity reaches your desired goal.