Start here.
http://melhuish.org/audio/horn.htmlRead the whole horn section and the calculators are the last things in the section you need. You can check the last pages in the section but they are just driver specific recommendations.
You only input 4 parameters, but it's metric. Throat area, flare frequency (lowest effective frequency), size factor (8 for cornerhorn), and desired width of cabinet.
It outputs 4 columns of information. The only ones I look at are distance from throat (length) and area, which is the cross sectional area.
This is the fastest and easiest way to model an ideal expansion rate. (Remember that variations from the plotted design are at your own risk. This makes no accomodations for shortened horns, hard bends or inaccuracies within the horn, bad room placement, etc., hornresp is better for that)
Use the theory you read on the previous few pages to design a loading scheme to couple to the throat.
If you want more good sources, just ask, I got lots of good articles on theory.