Form factor for rectangular shaped patch? [for radiosity]

Working on a simple radiosity demo, and im currently approximating all of my patches with disks, using the form factor:

A * cos (Theta_E) * cos (Theta_R)
---------------------------------
           PI * R^2 + A

(from nvidia paper) It works pretty well, except my patches are acually quads (not disks) so I get disk shaped artifacts when two surfaces are very close to each other. I am aware of the general point-polygon form factor, but it seems to give very strange results (not to mention is quite a bit more complex than the disk-disk) Does anyone know of a quick closed form form factor between two quads?