Al (outback). you are spot on, except. . . I need to grow longer thumbs for a couple of my bows.
Also, the term was likely in vogue when bow handles were essentially flat, ie English longbow. I guess I can put my hand against the string and insert my thumb into the locator cut out. or measure at another location, likely above the shelf. Doesn't matter where you measure it, so long as you always measure the same place.
I tend to make certain arrows for certain bows and have something. . a band on the cap dip, the end of the feathers,. . something. . that marks the correct brace height on the arrow, so I put it on the string and look down to check it.
ChuckC