Every board has to be measured in thousands of an inch for thickness and then the angle of the cut has to be set in thousands of an inch at a different angle to correspond . I have a woodworking discussion site of my own with over a dozen different forums . One forum is called Ringmaster Lathe Turning . It is the most comprehensive discussion and show and tell board on the entire internet on this particular subject of making bowls from a flat board.
I have also made a bunch of videos that show pretty well every step of the process from start to finish . Here is one of my videos showing how a flat board is cut into rings to form a bowl in five minutes and I have others that show how it is sanded smooth enough to 400 grit and beyond for finishing also in five minutes per bowl .
The Ringmaster has been around for a long time but there has never been a lot of discussion on it so some would buy one years ago and hardly ever use it because they didn't have much instruction to go by . I like to think I have helped out many with this in the past two years since I got mine and have made well over 300 bowls so far. It is an exacting art and does take a little practice at the start but like all things it becomes easy when you know how . . lol . .
https://www.youtube.com/watch?v=Pes0Qj8npms