Actually you are trying to keep your actor parallel to the local plane. You need 3 points from the plane, not two, for your problem (lines are defined by two points and planes by three). The point you are looking for is not from point A to BC but from A to BCD. The center point for BCD is (B+C+D)/3
Here’s an article that might give you a sense of what’s going on (you are trying to keep your ship at right angles to the local normal):