I run my 300w full spectrum LED at 15 - 20cm (5.9 - 7.9 inches) from the top of the tallest plant.. At 10 cm (4 inches) it starts slowly damaging the leaves.
- My seedling is around 40 cm (15.5 inches) from the lamp, and not stretching at all.
I'd say you could go a lot closer than you currently are, and turn it to full power as well. All you really need to do, is make sure it's not burning the plant, which an LED generally need to get pretty close to do. You can check that by hand. If you can hold it where your plant is, for a couple of minutes, without it starting to feel hot, you're good.