18" is usually what they stamp on the box or set of instruction. Depending on where you got the light from, it's either bullshit or accurate...it's a complete coin toss. Good general rule of thumb is to start at 24-36" to start. Your plant will let you know. With LED lights it's hard to gauge distance since the light is focused straight down compared to HPS, CFL or even regular Edison incandescent bulbs. Rwise is right, you can always cover up the stretchy stalk and it'll be fine. I noticed on the picture that the little seedling is in a huge pot. Be very careful in how you water it. Tersky sent me a great link on which pot to use, how to water in a big pot etc. Not a bad idea to check it out. It's on the website so you should be able to find it easily enough. Good rule of thumb is when the leaves grow out over the edge of the pot, it's time to upgrade to a bigger pot. Seedlings do well in a small solo cup, then to a one gallon, then to a 3 gallon. The final home for me is usually a 5 gallon (though this time I'm thinking about a 4 gallon). The final size of the pot depends on the size you want your plant to be. Just like a fish, it'll grow proportionately to it's environment.