LEC / CMH lights, how far from a plant should an CMH bulb be?

Asking this to the more experienced people using LEC/CMH. Just got my 315W CMH kit today and I was wondering this. How far do you guys keep your cmh lights from your plants? At the moment I have them 30cm afar (I am using a cooltube) and my temps seem to be fine but not sure if that distance could burn them. Plants are 3 week old. Any help appreciated.

Put your hand above the plants and feel the heat, then adjust accordingly

An expansion of the above statement - put the back of your hand under the light so it gets uncomfortably hot, slowly move your hand away until it just warm - that's minimum distance.

Have fun