Afghan Day 37 underwatering?

So this is my first grow and i'm worried that i am under/over watering but wasnt sure if im just being paranoid. I have an afghan growing, this is a pic at day 37. Its under an hps with a medium composed of Vermiculite, peet moss, peralite. The ph is right on 6.8 temp is between 70-80, humidity is usually around 40 and its under a 600 watt HPS. I watered it around 600 ml with foxfarm nutes. This is day two after watering and the medium was bone dry. At this stage how much should i be watering? Also, how is the health looking overall. Its got a great green color and untill today was perky. Any advise would help. And if your wondering there is a Great White in teh background at 36 days

Typo on the original post i meant to say the great whites are at day 26

I usually try to put my finger in the soil and feel for moisture before i water and that NICE little girl looks like it needs some water