That information is much what I used when I was figuring it out. I wish I would have went with a 5 degree cutter rather than the 11 degree. In order for me to clean up the forcing cone, I had to widen the mouth more than I wanted. If I'd went with a 5 degree, it would have just cleaned up what was there with minimal stock removal.
With all due respect to Mr Iowgan, He has it backwards that a shallower angle would cause a compound cut. If you have an 8 degree and use a 5 degree cutter, then you deepen the forcing cone, and when you reach the full diameter at the mouth, its longer, and won't have a compound angle. Contrary, if you have an 8 degree and use an 11 degree, you will have to cut it much wider to eliminate the compound cut. So with that scenario, if you stop cutting when you see you're as wide as the existing forcing cone, and look, you see a compound angle, and you have no choice but to keep going and cut it wider to remove the compound angle.
I am not an experienced gunsmith like Iowegan is, but I suspect he got that backwards in his mind and wrote it wrong, because when I used this advice to cut a forcing cone I ran into this.
I tried this primarily on a GP100 that was leading. I never figured it out either btw.