Water? Better lack of it than it's excess.
Not sure, just let it droop and you'll see. Living near a river where the air is moister than the average (75%, 2 weeks without rain), I'm watering my outdoors flowers once a week, consider the fact that they're 8 hours of direct sunlight and more than 25°C during the day and their leaves are not wilting, drooping or dropping. I could wait for more, but I already know what I'm growing, 3 gals containers. My vegs see no water for more than 10 days now, 2 gals pots, except for the clones in very small containers which waters every other day.
Sativa's and Indicas has different needs. Your medium matters. Turfa, perlite, sand, regarding of your composition it'll retain more or less water. Size of your container matters.
So many variables here. There's no recipe, just don't believe it. The best approach is still empirical, learn, ask, watch your grass grows.
I'll gather infos about hydrometer, but looks like the best approach to be sure (new growers) of how it should work.