A followup question...

After running the numbers with various horns, and looking at sometimes published cut-off frequencies, I am surprised at how low some small horns supposedly go. But in actual application, I see that HF horns are seldom used near their low end cutoff.

One horn tweeter I have in my commercially mfg'd PA spkrs, the flare has a published cutoff of 780 hz, and programs I have run the numbers come in close to that. This horn is used with a 12 db crossover at 2500 hz.

Is there a "rule of thumb" in this? For example, if a horn has a 500 hz cutoff, how much higher would be an appropriate crossover frequency? Or, conversely, if I had a driver which I wanted to crossover at 1000 hz (to pick a nice round number out of thin air), what cutoff would I look for in a flare?