Hi DeanThe "Quadratic Throat Waveguide" paper published by Peavy will answer almost all the questions you asked. Please download http://www.peavey.com/media/pdf/aa/qwp1.pdf
Another referance which may be of interest to you is the post "Characteristics of various horn flares" online at http://www.audioroundtable.com/PiSpeakers/messages/1623.html
Adrian