Yes you are correct. This is called a Included Angle. a 30 deg. counter sink has 15 deg. angled flutes that would be 15 deg on each side So 15 plus 15 = 30 included The same goes with a center drill. A center drill is a 60 deg. Included angle to fit a 60 deg lathe center. The center drill and the lathe center has 30 deg per side 30 + 30 = 60 Included
