All I can say here is trial and error. Bowls are just as complicated as short turns. Too little area and it's the choke point. Too much area in the wrong place and the air separates which causes other problems. The valve stem and valve guide are a pain to work around and get air around. The bowl also has to be designed to deal with the problems that the pushrod pinch creates. This is particularly evident on the cyl center side of the port wall and bowl. There are some crazy airflow patterns on that side.
In my experience the bowl just below the valve seat on the SM head does not need major enlargement. Depending on the size of the motor the bowl around the valve guide and the bowl at the apex may need substantial work.
Sorry to be so vague, but some of these things are hard to describe. And please read this as me sharing experiences, not giving advice. We've got Darin and a handful of other guys around here for giving advice.