I'm thinking about adding a 400watt HPS lamp to my setup. How far away from my week old girls should i keep the HPS? 8inchs? more less? Any input please.
alot mor then 8 inches hps gets super hot i would say a good rule of thumb is if its too hot for your hand its too hot for the plant if u want light close and ur vegging why not just get some cfls or t5 fluros i would just think running a 400 watt hps for 18/6 would get kinda expensive and what not but if u want go for it it wont hurt unless its too close lol good luck happy smoke
The age of your girls really doesn't matter. Far enough away so you don't burn them from heat. Check out my grow for some distances, everybody's set up is a little different though.
True enough about your hand. My 400w at 18/6 was probably $25 US a month. My 600w at 12/12 should be exactly the same.