(If somebody already wrote down this trick, I am sorry for repeating, I just haven't seen it) As far as there is no way to 100 % perfectly mirror shapes in the Livery Editor, there's a somewhat easy trick that might help. If you put a shape on the side and want it to appear on the same spot on the other side, simply max out both angle both stats in the projection settings. It then paints it all the way through the vehicle and the angle setting to the max makes it to be applied backwards on the other side. (It also saves you some probably much needed layers)
