400 HPS in a 20"x36"x63" Tent

According to the math and the pic, a 400 will barely cover the canopy area effectively.

The math:

According to the Pythagoras theorem, the distance from the center of the canopy to the corners of the canopy is 20.59".

(a^2+b^2=c^2)

Half the length of 36" is 18", and half the width of 20" is 10".

18^2+10^2=c^2

424=c^2
c=sqrt (424)
c= 20.59"

So from the center of the canopy to the corner of the canopy is 20.59". If the light is hanging 6" above the canopy the distance from the light to the corner (bud) is 20.59^2+6^2=c^2. c= 21.45".

The light is 6" away from the bud directly below it, and 21.45" away from the corner bud.

Looking at the following pic, the 400 is a perfect fit, as the usable range for a 400 is 6-21".

Good use of basic math to figure that out man. +rep. I will however say that people will use a 250w hps in a 3x3 area with okay results. a 400w will really kick ass in a space as small as yours. Just make sure you can handle the heat.