Bracing Height is measured from the pressure button to the bowstring at 90 degrees. Most bows will have a recommended brace height given by the manufacturer.
If the brace height is not known, then a rough guide for recurves is:-
70 inch bow = 8.50 to 9.75 inches,
68 inch bow = 8.25 to 9.50 inches,
66 inch bow = 8.00 to 9.25 inches.
64 inch bow = 7.75 to 9.00 inches.
The ideal brace height for a bow is one that
produces the quietest release with the least
amount of vibration in the bow.
In order to increase or reduce the bracing height by small amounts, twists can be added or removed from the bowstring respectively, but a string of approximately the right length must be used in the first instance. If you have no means of establishing the correct length of string, measure around the back of the bow from tip to tip, following the curves of the unstrung bow, then take 90 mm from that length as an estimate of length.