code: def _compute_std_0(self, c1, c2, mag):
          
          if mag < 5:
              return c1
          elif mag >= 5 and mag <= 7:
              return c1 + (c2 - c1) * (mag - 5) / 2
          else:
              return c2
docs: Common part of equations 27 and 28, pag 82.