LED grow lights

How close should LED grow lights be to seed starting mix/seeds when first planted and then as the plants grow? I'm concerned my plants are leggy. I planted igloo lettuce, tomatoes, cherry tomatoes, broccoli, cauliflower, romaine lettuce, bunching onions, red onions, walla walla onions, white onions and parsley. Some hasn't sprouted yet because there hasn't been enough time. I think the igloo lettuce looks good, but much of the rest seems leggy to me, but this is the first time I've done this, so maybe not. I try to keep the lights probably three inches away from the seed starting mix (close to the plastic dome until it's time for the dome to come off). It's difficult to keep the light the same distance from all the plants because one or two are taller than the rest in the seed starting tray. My trays have 24-36 cells. 

I keep the lights on about 15 hours per day with a small heater and fan running during the day. The average temp in the small room is 70-71 degrees F during the day. I run the oscillating fan on low 24/7. 

Thanks!