I'm doing digital painting (in Photoshop.)
I'm trying to do this study of a light against a wall, and it lights the wall, gradually getting darker as it moves away from the light.

I can do gradation easily with a pencil, but I'm having trouble doing it with painting, digital painting specifically. I tried using a few different pigments, planning for distance, and using the inverse square method, (isn't that the name of it?) Then I try to add a few different pigments in between. But then it looks very fake, (almost like a 90s adventure game with the very early pre-rendered CG artwork.) I've tried using the pressure sensative opacity, but when I use that it tends to look very splotchy, with some of the different opacity levels overlapping oddly. So then I tried using the gradation tool, but that doesn't seem to give me much control, at least not easily, and it appears to only work in one direction, whereas the light goes out in a round, circular way.

Does anybody know the best way of doing this?

Thank you.

(I think this is the right forum, but I'm not sure. I think it relates to a general technical art issue; gradation.)