A storm with a recorded precipitation of $11.0 \mathrm{~cm}$, as shown in the table, produced a direct run-off of $6.0 \mathrm{~cm}$.
| Time from start (hours) | $1$ | $2$ | $3$ | $4$ | $5$ | $6$ | $7$ | $8$ |
| Recorded cumulative precipitation$(\mathrm{cm})$ | $0.5$ | $1.5$ | $3.1$ | $5.5$ | $7.3$ | $8.9$ | $10.2$ | $11.0$ |
The $\emptyset$-index of this storm is $\_\_\_\_\_\_\_\_$ $\mathrm{cm} / \mathrm{hr}$ (rounded off to $2$ decimal places).