In my case, I added 16 to Impact Point Z because my Decal size Z is 32, and that way the decal will be the most intense.
There's one issue with the visual part, however. When you stand on the edge and move back and forth a bit, your shadow will jump up and down.
But otherwise everything works fine. Except for my decal image itself of course, I didn’t put much effort into that.
Additionally, you can add some conditions to that, e.g. only show the shadow when the character is not on the ground, or maybe adjust the shadow intensity depending on the character height over the ground, etc.