There is a "card catalog" very similar to what you find in a library that includes the locations for bends and correct angles for every car made. Also an inclinometer that attaches to the pipe to tell you what angle to make the bend. Of course, for custom work those things are usually not needed. That's how you learn though. Using the catalog, using the tools for the job. Once you get the hang of it, you can pretty much let the catalog collect dust, but it is still a priceless resource. Of course as with everything, the card system is now on disc.
http://www.bendpak.com/car-lifts/accessories/cs-1.aspx
