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

[Quaked54]

But is the way I am using this function correct?
This is a non static function that calculates the cross product of both vectors and returns a value.

@Quaked54

FVector ABase::CrossProduct(FVector Product1, FVector Product2) { ReturnProduct.CrossProduct(Product2, Product1).Normalize(); return ReturnProduct; }