Years ago, I bought a new longbow from Northern Mist. I probably don't remember all the details correctly, but I had a couple of conversations with Steve Turay about how he would recommend setting the brace height for one of his bows.
My understanding was since longbows are generally quiet bows to start with, Steve was in favor of getting the longest power stroke he could by lowering the brace height to help keep the arrow on the string as long as possible. But he said to watch two things. One is arrow flight, and the other is what I call 'wrist slap'. How low can you go before the bow string begins to hit the wrist of the bow arm.
If you are using a good arrow spine for the bow and your shooting form, variations in brace height won't likely have a major impact on arrow selection or noise, so why not go for the brace height that keeps the arrow on the string the longest. So the guidance was to start at 6.5-7.0" and keep lowering it until you feel the wrist slap then add some twists until the wrist slap stops. This is what I've been doing with ASL's for the last 10 years or so. My experience is I run into wrist slap problems before I see any arrow flight issues.
Not saying this is the best way or only way to determine brace height, but it's a way to figure out what works best for you versus chasing a number that works for someone else.