Well IMO more info is needed. You have a 250 watt MH. but are you using a reflector & if you have a reflector is it set up w/ a tempered glass bottom & a 6" (or 4" or whatever size) opening to hook up a duct fan? if so are you using a duct fan? all of this matters. if your reflector is set up so you can make a closed loop cooling fan on it then you can pull in fresh cool air from outside your grow room w/ some flexible ducting & put a duct fan on it (I reccomend getting a cheap on from home depot or lowes, a 6" fan costs about $30) and then go ahead & direct the heat out of your room w/ some more flexible ducting, insulated if possible or affordable. Anyway if you are cooling that way w/ a fan then it will keep your light much cooler & you should really set up this way. you can then get your light much closer to the plants & thus be getting many more lumens from your light. The further your light is from your plants the more lumens you lose. The other thing you should get is a thermometer. either just a cheap one or get in nice digital display deal that you can set up w/ mutiple sensors throughout your room. My understanding is your plants will love it about 75 degrees F. Anything over 90 and they are in trouble. So get a thermometer & that will be the easiest way to decide how far your light should be. As for right now, assuming you don't have a thermometer & since i don't know all the other variables such as how hot/cool your room is, your cooling set-up if any & what-not.... well I would say a good rule is that too far away is safer than too close. And I would say set your light up & put your hand 1/2 way between the light & the tops of your seedlings... if your hand feels rather warm then move it back. I'm not saying you should feel no heat on your hand 1/2 way betweeen but you don't wanna feel very much heat. Anyhow, my advice is err on the side of caution & don't have it too close, especially if they are seedlings. Then get yourself some good cooling set-up & a thermometer & you can get everything just right.