Hello there, I'm no expert on growing, I'm actually doing my first grow. I usually water them every 3 to 5 days, depending on the plants. There are several way to check the humidity on the soil. There are some probe that you can buy to check the moisture, another way is the pot lifting method. This is a simple solution, you lift the pot and remember the weight it has. After 2 day you lift again to pot to check if it feels lighter or not, if it is then you need to water, if not everything should be ok, check the other day. If you over water or underwater you will see the plant "dropping". If there is always too much water you risk of having roots rot.

As for the ferts; "Early stage flowering plants can often use a fair measure of nitrogen as they stretch. As flowering progresses, the plantā??s need for nitrogen diminishes. After a plant has been flowering for two or three weeks, we prefer to avoid any Nitrogen in our fertilizers." I personally use Earth Juice as they are organic. As for more advise on this you'll need to ask someone more experience... At least if you're not using Earth Juice, if you ever pick it up let me know I'll give you my recipe.

As for the the distance of the light from the top of you're plant with a 600W you should be all right at only 1.5 foot over them. There is my reference for that : Text

Hope this help a bit, let us know and post some pictures if you have the chance.