does anyone know a good height to keep my 315 cmh ...

315W grow lights should be kept at least 18″ from the top of your plants, with some models needing to be kept as far as 3 feet away. :muscle: good luck with her !

I use a CMH in the winter as I grow in the basement in Canada. They're very powerful! I would keep it at minimum 2ft. They give off a lot of UV and you could burn them quickly as seedlings. Start at 2ft or higher is my recommendation! Good luck.