A $5 \mathrm{~m} \times 5 \mathrm{~m}$ flat square roof receives $6 \mathrm{~mm}$ of rainfall. All of this water (and no other water) drains into an empty rain barrel. The rain barrel is in the shape of a cylinder with a diameter of $0.5 \mathrm{~m}$ and a height of $1 \mathrm{~m}$. Rounded to the nearest tenth of a percent, what percentage of the barrel will be full of water?