It doen't have to be bullet-proof! About 1/4 inch will do. Figure the seated length of your stern shaft bushing as a guide. If you're using a shouldered oilite bushing, the length under the shoulder is usually 1/4-3/8 inch. If your bushing isn't shouldered, you want to have the beef just under the bushing's length so that the screw can bear on the bushing's end. Anything thicker than the length of your shaft bushing is really a waste and eats into your linkage operating space.
"If you ignore the problem long enough, it will go away. Even flooding stops eventually!"