When in the day is the best time to test brix? At or before lights on or after?
If you want the highest readings, test when lights have been on for 2 hours. However, I find that I'm always taking readings when I prune, which is almost always in the dark cycle, due to my work schedule. As long as you take the readings at the same time you can make accurate comparisons.
Anything over 12 is going to be great. Once you get up to the 15's---my highest is 17----you know you've something very special.