Since there is some renewed interest in this...
I use Gimp, so I'm not sure of the relative function in PS, but Gimp has a shadow type called "Perspective Shadow"
which results in something like this:
You need to break each cell or row into layers, then apply the shadow setting the horizon distance to whatever the max is (48.?)
You can set the angle, length, blur parameters to create something pretty realistic.
This example uses a "down-left" light source, so you'd set it more "up-left" to match the RTP
Be Well