Your LED @ 15" will be fine. Do you know the wattage of each diode? Wattage dependent you won't want it much closer than 12" anyway, not b/c they will burn or bleach the plant but b/c of efficiencies gained from proper light spread. I keep my 3 watt diodes at 24" for starters and then move them down to 12" a week after the first set of true leaves appear. Note there are far wiser LED gurus here that can give you better guidance in this regard.

Concerning your seedling in soil, here is the standard recommendation. First, 5 drain holes are fine so long as they are big enough to serve the purpose. You can use a hot phillips head screw driver. Also put a couple of three holes in the side, low down. Add your seedling mix. Before planting flush with 2x water - 8oz cup = 16 oz flush. This is always good practice with any seedling mix no matter what the bag says. Let the medium drain thoroughly (1 hour?). It should still be thoroughly wet. Use a pencil to make a starter hole. Be VERY gentle handling the spout as any damage at this stage will likely cause death. Place your sprout such that the seed head is just below the top of the soil. Sprinkle in a bit of dried mix to fill the hole and cover the seed - it will only take a pinch. Then use a spray bottle set at a fine mist to water the seed into place. You can thoroughly wet it using the spray bottle. Add more mix if needed. You can cover with a humidity dome (cellophane) if you like but be sure to remove it as soon as the sprout pushes through the soil. If you use a humidity dome you probably will not need to re-water until after the spout appears. Even then you shouldn't require much water as the soil will be holding quite a bit from the flush. I use the spray bottle on seedlings 1x day during the first 2 - 3 days. Just enough to wet the surface with a little sinking in. Reduce water as the sprout becomes bigger and stronger. Over watering can lead to problems so if in doubt, leave it alone. Stretch the water progressively to encourage root development. Using an LED you will require less water than others who use standard lighting. By the 2nd week I am down to 1x every 4 - 6 days. And don't water until you get runoff until your little plant is well developed. There is no need. The roots aren't that developed. Water logging the medium just encourages issues like root rot and/or infestation. Keep it simple and remember, less is more. That and read all the stickies in the basic grow forum. When you're done that, read some more.