Your using a 400watt right? (MH or HPS?) 400's don't burn as hot as 600's and 1000's. Therefore they can be placed closer to plants, especialy if they are air cooled, have tempered glass, and are sealed up tight at the seams etc. 18 inches should be safe...I run a 400watt MH with an Agrosun bulb to add the red spectrum. My hood is 18 inches from the top of my tallest plant. The best way I know of to test heat is the back of the hand test. If you place your hand palm down under the light at the top of the canopy and it gets warm...the hood is too close. You want that hood as close as you can get it so lumens can penetrate to lower foilage. It's funny...your using a 400 watt and some T5's? Thats exactly what I have!