If you've attached a decal in code, then you can set the same properties you would in editor in code. Look at the decal actor code and see what you need to set it. I'm assuming this is an added component in the default properties area, so look there in the decal actor code.
Anyone care to set up a small example in code? basically the decal shouldn't penetrate the floor, but still be able to be cast down if the floor is far away.
Adding it to code would help a lot but in the case of your picture you are probably going to have to do more than that. You can't set the project distance very far as it would still go through the ground then, as you have now. If you make it too low you wouldn't see a shadow when jumping. What I would do is a decal that…
Adjust the Near and Far Planes in the Decal actor properties. You should see the white bounding box adjust in shape. There isn't a way I can figure. without coding, that would handle it in such a way you are asking however. As long as it's perfectly pointing down, it shouldn't smear along walls. Take a look at some of the…