To compute an infinite horn, enter "0" in the "ANG" field. This is best used to approximate the response of a large group of similar finite horns.To model the expansion from a trihedral corner, use the formula A=X230.5/4 to calculate area expansion, where A is area and X is distance from the apex.
If you have any problems using Hornresp, write me and I'll walk you through it. You can also contact David McBean, the author or the program. He is very approachable and is glad to hear from people using the program.