I have been trying to figure this out for myself and I cant seem to find the information I want in books. All I could find is this informaton:
The theory is that you are meant to have a venturi point going in from the valve to a choke point of 0.85 to 0.87 of the valve dimater inside the port and from that point outwards to the edge of the trumpet is meant to be one gradual angle increasing in size till the opening. Its all theory though and I couldnt find any real world testing however I should have another source of information coming soon that may have some more info on the subject.