Some people say that if you can hold your hand by the plant tops then it is not too hot. I disagree with this due to personal experience. I like to hang a thermometer over the light, at the top of the plants so it's directly under the light. It needs to be under 100 or you could have problems. Under 95 to be safe.

I was assuming those hairs were turning first because they are the closest to the brightest area in the grow. This happens all the time, but if it feels pretty hot under the light, I would check like I described. 8 or 9 inches sounds fine for 400 watts, but it's close to the borderline.