How far should I keep my 250w hps from the tops

Does anyone know how far i should be keepin my 250 from the plants. Max height and minimum height with no cooltube

Gday, ok the 250w light i run 2 of them in a grow tent split in half the lights limit is min six inch less will burn and maximum eighteen inch now what I do is put a string hanging down from my light with colored tape on it one at six inches one at twelve inches and tye something like a nut at end which is 18 inches or light limit. When i start seedlings or clones i use light outside its limits gor a week or so with mature plants try to keep around 12 inch mark for max light. Their i did all your homework for you now you owe someone else the weight is off me. You know there is a lot to learn about this shit i been at it since this January well peace to you my brother.