Well, the different methods for drawing perspective is just approximations of the true perspective. Look at it this way, perspective is really how objects relate to each other and the horizon. Now the thing is that the horizon is curved, right, so basically the wider panorama you paint the larger the number of vanishing points. There's more to it, but I don't know it. The easiest way is just to look and see; does it look like true perspective, or not. In this case, I say not. The upper edge on the facing wall would look better if it was tilted down by a few degrees towards the edge of the canvas, and that's why I think there should be a second point somewhere on the horizon.
So yeah, I hope that made some sense, it probably should have been included in my previous post. Sorry about that. *apologetic smile =>