how to come up with the cut line when working with tubular construction?

The question was long and so i made it into a PDF file for ease... I know that this isn't exactly a CAD question but being that this site is full of engineers it seemed like the best place to ask. The basic issue is that i have a method for coming up with a cut line when working on or with tubes that need to be put together (Primarily tubular chassis or frames). the question is is there an easier more practical way of doing what I'm doing or does anyone know of another way that gets me the same results. If anyone could take a look at the PDF and help me out that would be great thanks.