Like Vaughn said, Tibbett's book/DVD is a very good reference for segmented turners.
For simple segments, a table-saw sled works just fine.
For more precision and particularly for feature/design rings, a disc-sander jig is needed. I think Curt Theobald also uses a jig like that in his DVD
This is how I calculate segments without any software:
Chinese Proverb: Man who eats many prunes gets good run for the money.