i dont know the exact equasion, but my 150w is about 4-5 inches above and you do have to watch for burn, and 400w (ive read) should be less than 12 if you can do it without burning. so 250w? wait for a more expierienced grower though. and not that close on seedlings(i used flos) to tender still. veg is about 2 months and so is flowering, depending on your own situation. do some reading, most people just go from one stage to the other. not messing with the slow time change, 16/8 to 12/12.