The actual diameter of the aperture may be different from what the textbook number says, depending on the optical formula of the lens. I would think it is probably correct. Take a lens you know is ok, and do a test. Using a brightly lit wall, set the known lens on some f number, and take a reflective reading off the ground glass with your light meter. Replace the known lens with the lens in question, set it at the same f number of the control lens, and see if your light meter gives the same reading. This method is also used to calibrate a replacement shutter that has a blank aperture scale.