A screw gauge with a pitch of $\mathrm{0.5 }\mathrm{}\text{mm}$ and a circular scale with $50$ divisions is used to measure the thickness of a thin sheet of aluminum. Before starting the measurement, it is found that when the two jaws of the screw gauge are brought in contact, the ${45}^{\mathrm{t}\mathrm{h}}\mathrm{}$ division coincides with the main scale line and that the zero of the main scale is barely visible. What is the thickness (in $\mathrm{}\mathrm{m}\mathrm{m}$) of the sheet if the main scale reading is $\mathrm{0.5 }\mathrm{}\text{mm}$ and the ${30}^{\mathrm{t}\mathrm{h}}\mathrm{}$ division coincides with the main scale line?