Hi, just ran across an answer that worked from jangsy5. In my case, I had to add 180 to the result, but then it worked for me.
((2 * wall.Angle) - Player.CustomMovement.MovingAngle) + 180
My ship is "Player" and the wall object is "Wall". Originality at its finest. When the player collides with the wall, it calls the Custom Movement behavior, to set the angle of motion to the formula above.