Here's an image I did early on the ArtRage alpha testing. It's a means to use the rulers to do two-point perspective.
Create three rulers. Move a ruler to the far edge of your zoomed out canvas, and pin it to you 'horizon line', with the pin touching the edge of the ruler. This is one vanising-point.
Repeat at the other edge.
Now you can use the grip at the other end of the ruler to quickly position them for drawing construction lines for perspective.
Use the third ruler vertically - just right-click drag the ruler quickly to position for vertical lines.