Solved
O.K.
There's more than one way to do this but the following is about as simple as it gets. Set up a Datum Plane that bisects your quarter bowl, then create an Edit/Intersect between it and the outside surface of the bowl. This will work best if you pick the outside surface first. (This will usually mean selecting on it twice, once to get the solid feature and a second time to get just the surface.) Now Ctrl-select the Datum Plane and perform the Intersect. You can also do this by projecting a straight line onto the surface, but that's a bit more complicated.
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