How close can I get my 250w HPS to my canopy ?? (HELP!)

I have a 250w hps... i wanna get it as close to my 2 clones as I can for optimum growth but i dont wanna burn my babies...whats the best way to know how high to put the lights above the canopy? I have them 36" away now.. bt I can put my hand about 20" from the light and not even feel any heat... if anyone has experience with a 250w hps over a couple plants lemme know how close to get it

get it as close as possible without burning them , if it dosent feel hot on your hand then it will be ok for the plants , its a case of trial and error really , a good fan blowing across the tops of the plants helps alot , and of course you need your intake and exhaust to keep cool fresh air coming into your grow space .