For those who go with mitered tubes - this might help
 

I'm using mitered tubes on my base and to make my miters/fish mouths I use this simple application to print out templates that are affixed to the tube and then cut with my plasma cutter. It's in metric but if you convert your inch tube measurements to metric, it works fine.

http://www.ozhpv.org.au/shed/tubemiter.htm

It's completely free. If anyone knows of another inch version - please let me know. A web-based version would be excellent.

Thanks,
david

Here's a web-based version for dust collection pipes, that could probably be used for the miters as well.

For calculating the fit to a flat surface, I would try setting the main pipe external diameter to a very large number.

http://harderwoods.com/pipe.html

Michel