In all the woodturning plans I see there is a column for board width. For example:

http://www.segmentedturning.com/imag...hemisphere.gif

How is this figured? I know how to figure angle based on number of segments and length of segment based on circumference. But I don't know how to figure the width of the board used for each ring so that I don't end up with too thin a spot once glued up. Is this figured from just drawing it up or is there a math component? I want to just put together a spreadsheet for this rather then buy software to do it.

Thanks,
Jim