Standard practice is to put the canter placement rail 1 stride out from the base of the fence. If the horse needs to adjust, they adjust to the pole, thereby setting themselves up correctly for the fence.
You can add a placement rail on the landing side for horses that hollow or rush in the landing.
Placing a rail 2, 2 1/3 or 3 strides out from the base really isn't going to help the green horse find his distance, there are too many things that can happen between the rail and the fence. If the horse is so dead solid steady and consistent in his canter that nothing changes from 3 strides out, he no longer needs the placement rail.