Matt,
You adjust brace height be lengthening or shortening the string. If you twist the string and make it shorter, in effect, when you restring the bow, you pull the two limbs closer and move the string further out from the bow, thus increasing the brace height. Lengthening the string by untwisting will shorten the brace height.
The shorter the brace height, the longer your arrow is in contact with the string during release. This results in a longer "power stroke" and more speed. The increase in speed is likely not that great, maybe a few feet per second, though I have never actually measured this.
You can get a pretty good idea of what brace height your set-up likes fairly quickly by setting your brace height at about 7.5 inches. Shoot several arrows, making a mental note of noise and arrow flight. Then reset your brace height to about 7.75 inches. Shoot several arrows. Is it quieter and do your arrows fly any better? Set your brace height to 8 inches. Shoot several arrows. How does your bow sound, and how are your arrows flying? You are looking for the best arrow flight with the least amount of noise.
Noise and arrow flight will ultimately decide the best brace height for your set-up.
Once you find out roughly what range of brace height seems to work best, you can fine tune your set-up by making 1/8-inch changes in brace height over a smaller range. Make sure that you are comfortable you have good shooting form, because your ideal brace height may be something of a moving target until you have good form.
My longbows tend like about a 7-inch brace height. I have several recurves. They tend to shoot best at brace heights ranging from just under 8 inches to about 8.5 inches.
Good luck!