$$\alpha = \begin{cases} 0.673 & \text{if m = 16} \\ 0.697 & \text{if m = 32} \\ 0.709 & \text{if m = 64} \\ \frac{0.7213}{1 + \frac{1.079}{m}} & \text{if m $\ge$ 128} \end{cases}$$