You do use keratometer (curvature) readings of the eye as a base to start when selecting a lens. Other factors that matter are the size of the cornea and the particular characteristics and dimensions of the lens you are working with. Some lens companies like Ciba Vision try to take a one size fits all approach, since many of their contacts do come in one base curve. However, sadly, they do not fit everyone. Vistakon's (Acuvue) pretty much come in 2 sizes and base curves, except their multifocal and toric lenses. So their lenses do fit more people. Tne negative here--higher cost of doing business and maintaining inventory. Another example, even though both the 8.3/14.0 Advance and Acuvue 2 have the same nominal measurements, the 8.3 Acuvue 2 can fit a steeper cornea than the Advance because of the stiffer modulus of the Advance--means it doesn't drape the cornea as well. Even people with same corneas (curvature) but different widths (corneal diameter) may end up needing a different lens.