[Muzman]

How's it done usually?
Any theory out there, or is it an educated guestimate?

Impart, oh numerous wise ones.

[Derek]

The best means I can use to try to describe doing this with is one-point perspective but 'in reverse'... if you work out a station point in a perspective drawing, the light source will be at the SP (if you are using a single source to plot your shadows).
If you have no idea what I mean... then start by choosing the area where according to your drawing or painting so far that the viewer is standing, sitting, flying, etc...
Then from that singular point, extend lines outward to your objects-keeping them on the same ground plane.
If you have multiple light sources, plot the work on the ground plane or whatever plane the shadows will be extending, crossing etc. accordingly, but remember to be defferential in your sources, they should not all have the same strength, color, fall off, etc... and don't plop your viewer in the middle of the scene. You will need to create a lighting hierarchy.
There's a lot to getting this 'nailed' but a good guesstimate is more than possible.

[Muzman]

Cheers D.S. very interesting
(no other takers? where all the technical folk go? Not that I'm dismissing your's Derek. I'm just polling.)