This is just what I would do... you may want to do differently.
I would take a cut(s) 1 to 1-1/2" on each side of the pith. This will leave you with two sections 6-7 inches thick which for me is plenty deep for bowl even losing a couple of inches from the bark side. You can mark and measure but the cut side should be about 13-14 wide.
You will have a center section 2-3 inches thick 16" wide and I assume 21" long. Again cut it down the pith giving you two section for shallow bowls, winged bowls or lots of other stuff.
In general if you remove about 3/4" of the pith area you should be fine.
If you go here there is a sample of Richard Raffan's book. About page 15 may give you some ideas.
I am assuming you will cut to match the grain on each side of the pith so you may come out with one larger and one smaller section.
Hope this makes sense.
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Last edited by NCPaladin; 03-24-2012 at 08:42 PM.
Reason: added link