In a trading manual I got during a course, the value of the ATM Call-Spread is approximated by $CS_{ATM}=\frac{1}{2}StrD+(F-m)\times\Delta CS$ The lecturer skipped the part where he derived this approximation. And couldn't answer why this formula holds. So does anyone have a clue? StdrD=Strike Difference, m= midpoint between the strikes of the call spread F=future
$\Delta CS$ was approximated by $0.33\times\frac{StrD}{Straddle}$ (which is a consequence of the normal distribution) Where we take straddle equal to a standard deviation (actually $\sqrt(\frac{2}{\pi})$ times would be more precise.