A histogram is constructed for a dataset of $N=500$ observations using the Freedman-Diaconis rule to determine bin width $h = \frac{2 \cdot IQR}{\sqrt[3]{N}}$. If the first quartile is $Q_1 = 40$ and the third quartile is $Q_3 = 80$, what is the optimal bin width $h$ (rounded to two decimal places)?