The rigid bar remains perfectly straight when it deflects. Therefore, the deflection at $\mathrm{B}$ can be determined, using the principle of similar triangles, from the measured rigid bar deflection at $\mathrm{C}$.

$$\begin{gathered} \frac{{\mathrm{v}}_{\mathrm{B}}}{400 \mathrm{mm}}=\frac{2.52 \mathrm{mm}}{1,200 \mathrm{mm}} \\ \therefore {\mathrm{v}}_{\mathrm{B}}=0.84 \mathrm{mm} \end{gathered}$$