I did some checking around, and best advice i could find was this.
"Start at eighteen inches and move it down a few inches a day until it is within a foot of the canopy. The plants will grow up to meet the light and you don't want them to stretch too much."
You might also find this link helpful. It has a calculator to help you determine exactly how far they should be.
high pressure sodium grow lights page
about half way down the page it has Distance From Plants section.
Hope this helps.