A slab panel with an effective depth of $250 \mathrm{~mm}$ is reinforced with $0.2 \%$ main reinforcement using $8 \mathrm{~mm}$ diameter steel bars. The uniform center-to-center spacing (in $\mathrm{mm}$ ) at which the $8 \mathrm{~mm}$ diameter bars are placed in the slab panel is $\_\_\_\_\_\_$. (rounded off to the nearest integer).