How to change Decal's rotation at runtime relative to the floor or wall? C++

[Quaked54]

I appreciate the in-depth explanation. I feel more enlightened on the topic at hand.

@jwatte

if (fabsf(impactNormal.Z) > 0.8)
 {
    basis = FVector(1, 0, 0);
}

With this function you are trying to get the absolute value of the impactNormal’s Z axis and checking if that absolute value is greater then 0.8? Then you change the basis vector’s X axis based on that. You are checking to see if the Z axis of the vector is greater then 0.80. But why 0.80 in particular? Please correct me in anything that I am saying.