You just try to get as much of the theory down as possible and start building them. So far the best theory I have found was my own through lots of trial and error. If you don't want to get into it there are a few guys out there building them for other people. The biggest problem I have found with most programs and formulas is that they are designed for bikes.
This might be the wrong way to do it but the best I have done is to copy some successful pipes (from pictures), threw in some trial and error, and came up with my own formulas. It will be tough to get someone to give you the answers your looking for.