Watering (how much)

Hey everyone. Me and roommates are growing 3 feminized white rhino plants under a 400watt metal halide lamp. We are using a soiless mix and using nutrients once a week. I believe they are in 3 gallon pots. There are pictures. Take a look, let me know what you think.

I have been giving the larger plants a little bit less than a half gallon with nutrients. The moisture meter is near bottom of the soil and says it is below moist the day after watering. I am using a 400Watt Metal Halide 20" away and it is 77 degrees in the room. Should I water them when the soil gets dry throughout the entire pot near bottom? Or just a finger length deep?