\Phi\left(\frac{n - \frac{n}{4}}{\frac{\sqrt{3n}}{2}}\right) - \Phi\left(\frac{500-n-\frac{n}{4}}{\frac{\sqrt{3n}}{2}}\right) \approx1 - \Phi\left(\frac{5}{2} \cdot \frac{400-n}{\sqrt{3n}}\right)