Effective range for 150w and 250w hps?

What are the distances plants should be from 150w and 250w hps lights?

not positive, but my 100w HPS light is hanging around 18" above the tops of my tiny little sprouts. that seems to be the number than someone else gave me as well.

the way i use to tell if the lights are too close is by lowering the lights as close as possible to the plants and i then put my hand directly above the plants and if it feals too hot to keep it there, then the lights are to close. Its a good idea to do this every day or so and make the neccesary adjustments.

Lumens is a mathematical formula that measures the bulbs intensity at 1 foot. The color or type of bulb is not calculated in the formula I=L/D²
Lumens received are a measured in watts-per-square foot or foot-candles. Calculating foot-candles or lux is a more accurate way to estimate the amount of light plants receive. This still only measures how much light you plants have available but not how much your plants use.
Light diminishes exponentially fast!
The closer your plants are to the light the more lumens they receive the better they will grow, as long as the heat from the light does not burn the foliage.

I keep my 250w HPS about 6 inches away from the top of the plants but only because I have a fan blowing directly at the bulb. If there was no fan to cool it down I'd have to keep it a few inches higher.
When I use my metal halide bulb I keep the lamp at least a foot away from the plants even with the fan on because the metal halide gets much hotter than the HPS for some reason.

thanks for the info