A theodolite is set up at station $A$. The $RL$ of instrument axis is $212.250\: m$. The angle of elevation to the top of a $4\: m$ long staff, held vertical at station $B$, is $7^{\circ}$.The horizontal distance between stations $A$ and $B$ is $400\: m$ . Neglecting the errors due to curvature of earth and refraction, the $RL$ (in $m,$ round off to three decimal places) of station $B$ is ________.