When using those plastic swing arm hydrometers it's best to keep your SG
in the middle of the acceptable range. 1.024 is a good number to be at w/ one. That way if the hydrometer is off you will not be too high or low. Basically leave some room for error if you doubt your equipment at all. Be sure to wash it after every use w/ fresh water and measure on a level surface otherwise they can be very misleading. FWIW my two IO hydrometers where only off by .001 when compared to a refractometer.
After you install your lights you may have to adjust your temp to avoid large swings between night and day. If your lights heat the tank during the day, which is an issue more w/ high light systems esp. those w/ MH
, you will have to adjust your heaters so the tank doesn't cool too much at night. Keep temp swings under 2 degrees daily. 77-84 is fine, as long as it's stable. IMO
80-81 is ideal.