Start with the 7-1/2" brace hight and then twist or untwist the string to try various brace heights until you find the "sweet spot". That is tuning the bow.
Different factors will effect what brace height is best. The spine of the arrows you shoot, your release style, your draw and anchor style.
You may also need to experiment with moving your nocking point up and down. Start about 1/4" above the arrow rest and play with it from there.