I know exactly what you mean, I have a board bow I built a few years back. Shoots pretty nice,
I did set the string knock point with a square, then I walked the knock up and down till I fine tuned it.
Regardless of the yardage I shoot at, up to 40-50 yards, I can group arrows about 4'' diameter.
So I guess now, I need to note the position of the string knock, and then twist and untwist the string and see if I can dial it in better.
If the grouping get better, then note the brace height.
Does this sound about right?