Tangie

Hi GRONE, Depends a bit on the temperature at the top of the plants. If you use a cool tube you might be able to get it just a bit closer as it won't burn the plants. When I used HPS I never went below 50cm. If you use CO2 on your tent you can lower more than that as the plants will be able to handle higher light intensity. Check list: Temperature at top of plants first. If it's not too high adjust height. If you can make a reading of PPFD map you might want to keep it around 900 in flowering. Adjust height according to that. If you have CO2 you can increase the PPFD meaning you can adjust the little further below. Here's a link with a little explanation by Mars Hydro (not sponsored). https://www.mars-hydro.com/info/what-ppfd-for-weed

That really depends on how healthy your plants are and how good ventilation you have. If you have really healthy plants you can put the lights as close as 16 inches or even a bit less without any problem.

When I run at 600 watts I keep my light 18 inches from the plants