How far to put a 250watt MH from plant

I am wondering how far to put my 250watt MH from the play I had it just under 18in and the tips of the leafs started to roast a little is there a sent distance or should i just keep moving it around?

well mine is air cooled and is 6 inches away with no burning

I have a 400w mh and it is air cooled by a box I made with 4" dryer duct and a fan. I use a digital cooking thermometer with a probe to keep track of the temps. I find it works better than a regular thermometer. I can put the probe wherever I want, even right down in the soil to check it. It also has an alarm you can set to go off at a certain temps. I recommend getting a thermometer of some type to check temps at the tops of the plants instead of waiting till it burns to find out its too close.

Use the back of your hand. Put it near the tops of your plants and if its comfortable for your hand, then its good for your plants. My lights are normally about 8-12 inches away. 400w MH conversion and HPS is my light.