You can get T5s within a couple of inches usually - but with seedlings it's fine to back them off a bit as long as they aren't stretching. But actually, if that is a 150w HPS (looks like it), you can probably get that a lot closer even with seedlings. I usually start a 150w HPS about 12" away over seedlings at first but move it down a couple of inches every day or two until within about 4 inches (as long as you keep an eye on temperature). There's about a 40,000 lumen difference between 4" and 12" with a 150w HPS. It makes a big difference to get it as close as you can.