Get it as close as you can without burning the plants. I lower my light to what I think is right, then keep an eye on things for a few days. If the plants are doing fine, I'll lower it another inch. At some point, the upper most leaves will show stress, then I back it off a little. There are a lot of variables, including age of plant (younger plants are more tender), air flow in room, temperature, humidity, age of your bulb, and the strain you are growing. Proper light distance is key for good results.
Right now, I've got my 1kw MH at 24" on 18 day old plants. I'll leave it there for a few more weeks, but once the plants get tougher, I'll increase the airflow, and shoot for 18-20". I know this light really well, and that's the sweet spot for it.
If I were you, I'd even your canopy out with some LST. You've got some nice plants there. Even that canopy out so you can get the light close, and you will get a good yield.
