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Temperature is really the only determining factor regarding the distance one places the lamps from the plants, excluding, of course, the possibility of light intensity being high enough that light bleaching occurs. Start with a general rule like coda posted and make adjustments depending on the temperature of the canopy. The hand test, like he also posted, works well, and of course a thermometer will work as well.
If the reflector for your lamp is air-cooled, this will change the distance the light can be from the plants, and having a fan blowing air across the canopy underneath the lamp will help too. The closer you can get the light to the canopy without the temperature becoming too hot, the better (generally, as it is possible to have too much light intensity on the plants as mentioned above), since not doing so is only wasting light that the plant could be using (inverse-square law).
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