You should gain experience with what compression ratio to use given the subwoofer type and size, and other parameters. Read a guide at avs forum and learn to use horn resp and that will get you going.
I am new to horns to be honest.
Horns try to couple the subwoofer to the air, because normal boxes make it to where the subwoofer could care less about the air's coupling to the driver. The combo of motor of the subwoofer and subwoofer's parts couple their self with eachother and the air around the subwoofer does not couple good to the subwoofer because the air does NOT really affect the subwoofer's impedance like the cone and spider do, impedance is legitimate and allows coupling between things if it's matched.
So don't listen when people claim that impedance rise is bad, it is apart of the process.