My opinion and experience (for what it's worth) is that throat size is a result, not a goal in and of itself. The size of the throat is a result of getting the bottom cut angles and blending to direct the air where you want it to go. I have also found that the throat size can be used to regulate flow separation to some extent. Bottom cut angles and blends will be manipulated based on the port in front of the throat and the chamber after the throat. To say a throat needs to be X% is just incorrect in my way of thinking.