keep it as close as possible, just dont burn em. make sure its not close enough to burn, and you'll be fine.
I was about to ask the same question. I to have a 250 watt. Mine is going to start at 20" away since I read in another thread where someone said to keep a 600 watt 30" away from seedlings.
I have a 600w and keep it 2 feet away from seedlings if u just put em in soil unless if few days older then lower it but not till 10 days old minimum for it to be close, and so 20" as a test height? and my clones and lil babys are about 2 and a half weeks old