I have found that its best to keep the light about 24in above the plant in the first week (when the plants are about 2in high or the first week you transplant clones) Then 12-14in above your plants during the rest of veg with the top of your tallest plant being 12in away. You want to try to keep your canopy as even as possible hopefully with just a few inches difference in your plants. During flower I usually like to put the light at around 10in and let it grow to 9in then move it again. It also depends on if your light is air cooled and what the temperature of your canopy is when you have your light that close. If you had a digital thermometer with a probe hanging over the canopy you could see what the temperature is right above the canopy when you have your light at a given distance. You want to keep the light as close as possible without burning the plants or causing them to wilt from heat. Its optimal to have you canopy remain at around 72-75*F but it can vary from there. I wouldn't recommend temps below 60* or above 90*. If you can keep your temps in the 70s at all times that is optimal especially during flower. Night temps can be 10* lower than day temps without any problems. However, my friend has grown during the summer here in CA when its over 100*F during the day and his box got to 100* sometimes. Even with those conditions his buds still turned out dank. So its definitely possible to grow dank buds in less than perfect conditions. Hope that helps.