A professor of mine taught us a way to draw lines to vanishing points that went off the page without having to tape together three or four rulers to locate it. The first step was to draw a horizontal line, then on top of it draw a line at the angle you wanted; after that, my memory of the technique is extremely fuzzy.
I think part of it was to calculate the distance between the horizon and the corner formed by the horizontal/angle, then to find the distance between the horizon and some other part of the angle, then divide the answer at some point. I wish I could remember, because it's a very handy trick when you don't know perspective well enough to guess at extending lines from a VP.
Does anyone know what I'm talking about? Or, do you have a better way of doing this?