I needed a shaft with a cam on it to adjust play in a mechanism. A piece that looked about right to start with was a carriage rod from an old printer. It sure did machine nicely, and the cam was easy to make. I took a piece of sheet metal, put a bend in it of about 120 degrees, and put that between the rod and two of the jaws. With a little care to make sure the cutter cleared the work to start with, it went fine. (on my first try, I forgot about the rod being eccentrically chucked- the cutter dug in and I had to remount the rod) I turned it until I was just barely taking a continuous shaving off. I don't know if there's a formula for this, but I don't suppose it would be hard to figure out.
I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-