Assuming that the 600 watter is a blurple? If that is all you have, put your LED between 34" - 36" above the top of the little seedling. Give it a few days to see if the plant is showing upward growth. Then gradually lower the light an inch or so every day until that distance to the canopy is 22". I'll leave the blurple at 22" above throughout the VEG cycle.