Actually, I advise switching to flower sooner. Here's why.
Your HPS600 is going to penetrate the canopy in only a limited way; once you pass that, you are putting on wood unnecessarily. If you want to veg longer, I strongly suggest training your plants so that they grow rather flat and low, and the light penetration is to MORE of the overall mass of the plant.

Gravel in the pot is not necessary. Info for next time. Light soil with perlite, and large drain holes, are all you really need.

Light distance- totally depends on a lot of things; bring the light as close as you can before you hand held palm-down at the canopy level feels warm but not uncomfortably so. Or if you see the edges of the higher leaves curling subtly but sharply upwards, raise the light. And remember that air movement is very important!

I can't give you # of gallons per week either; totally depends on your plants, conditions, soil, humidity, all that junk. To runoff is totally fine as long as you are giving ample food and letting the pots dry out a bit between waterings.

Um.. that's it I think? If I missed anything... post up!

Edit: What do you have against tan? It's an overlooked little function, with a low self-esteem, and offended that you left it out!