Thanks, but that was not what I was looking for.
In case someone else needs to know, the general formula is:
C = 180+2B-A
Where A is the direction of incidence, B is the direction of tangent (0 if horizontal, 90 if vertical) and C is the direction of reflection.
An Example Scenario:
A particle moves at 32° and collides with a wall. This wall is rectangular in nature, which means that there is both horizontal and vertical sides.
On a horizontal tangent, it would work like this (sorry for ugly, not-to-scale diagrams)...