Establishing perspective in ref photos?

[JQP]

(I'm very open to alternative techniques)

So, I can use the line tool to overlay lines in the ref, but in order to extend the lines far enough to hit the horizon line to get my vanishing points I have to zoom way out. So, I lose accuracy. Is there a way to draw a straight line segment, then automatically extend it in one or both directions to infinity?

[Latu]

You could use the pen tool to draw the lines and free tranform it and enlarge the path, effectively extending the length of it, hold shift while scaling so you keep the angle then line it back up with the edge you were using to establish the v point. Remember workpath is only temporary, rename to make permenant, otherwise if your in layers mode and start a new path you'll lose the work path layer. Oh and stroke your paths yada yada yada so on and so forth.