As with all lights, you want your plants to be as close as possible while still being able to keep the air temperature around the plant in the acceptable range (anything over 85 degrees F will inhibit growth). This depends on your setup and location. A good distance to try for is a foot, although you will have to keep an eye on temperatures to make sure this isn't too hot a region. A plant will use as many lumens as it can get.
i read awhile ago how much a plant actully uses when its in the flowering stage. i just forgot what it said.. like it there a certain number of plants that i can grow under the 250 without all the other plants being afficted by too little lumens per plant?
It's better to estimate by wattage rather than lumens, if only because different bulbs can give out different amounts, and you won't always know how much a bulb gives. The rule of thumb is 100 watts per plant, and with that standard I doubt that you'll be giving your plants more lumens than they can absorb. So with a 250 watt light alone, you'll be able to easily grow 2 plants. You could probably even get away with 3, but I would add some CFL's or something to maintain that 100 watt/plant ratio.
I use the tried and true "hand tecnique". If you can feel the heat on the back of your hand , so can your plants. Place light just past the point of uncomfortable heat on your hand and it will be just right for the plants.( My garden has 150,000 lumens in the flower room and right now 9 plants loving all that light.) IMO Much Love.
Ive seen people do 4-6 with confined spaces and hydro not soil with 250w. Im going to be doing 4 with a 250w. Not alot of heat either!!
It's not like with less than 100 watts per plant they'll explode and you'll have nothing. With less than optimum light they won't grow as well as they otherwise would, that's all. With an HPS you can probably get away with less than 100 watts per plant without too much loss because the light is more intense than, say, a CFL. But if you stick with the guideline, you won't be disappointed.