With a 1K HPS you have about a 36" optimum penetration range for the light which is killer. To me it depends a lot on how many plants you are going to flower under that one light. I have a 1K hps and when I do grows from seed in search of a great pheno of a strain to add to my grow schedule, I'll keep around 8 female plants max and flower them around 15-17" from the dirt. I also remove the lower 'sucker' shoots from the mainstem from about 1/3 to 1/2 way to the bottom of the plant to force all budding hopefully withing 36" of my light, with the closest plants being about 10" to a foot from my air cooled hood. If I am growing a known strain I'll veg larger and transplant them into 5, 7, or 10 gal pots and do 3-4 plants at a time.