I've come up with a new layout.

The idea is to have your main across the top of the screen, your healer occupying three quarters of the remainder of the screen, and your DPS sharing the remaining section. It does this whilst retaining aspect ratio across all five windows.

Here are the coordinates for a 1440x900 screen:

Window 1 (main):

Position: 0,0
Size: 1440x512

Window 2 (healer):

Position: 0,512
Size: 1080x384

Window 3 (DPS):

Position: 1080,512
Size: 360x128

Window 4 (DPS):

Position: 1080,640
Size: 360x128

Window 5 (DPS):

Position: 1080,768
Size: 360x128