250 WATT MH and HPS questions

So i just got my new lights and i have a couple q's

1st what is a good distance between plant and bulb

i dont have a reflector but its been just fine without it but one is on the way

now has anyone used a 250 watt is there a big yeild differance will it take longer to grow or am i just paranoid

you'll have to measure the temps at the canopy level. lamp as close as you can while keeping temps below 85F... 29C

ty vm man ill go do that now
my book says 16 to 18 inches but i just seems too far

Distance vs spread.

As the light distance increases for the tops of the plant, the intensity decreases, but the spread of that decreased light increases.

If you are only interested in the center cola of the plant for budding, go close as the heat will allow with a quality reflector. If more branches are to bloom then back up a little. Other side light might be needed depending on the room configuration. You need to reach more then the very top of a plant.

thank you all very helpfull