You measure from string groove to string groove and subtract 3 inches for a longbow and 4 inches for a recurve.
Here is the AMO Standard:
A Traditional bow is measured from string groove to string groove around the curve of the bows limbs, not directly from tip to tip. This measurement does not go into the curve of the handle but across the face of the bow limbs from string groove to string grove. This will give you the approximate length of the bow.