A theodolite is set up at station A and a $3$ m long staff is held vertically at station B. The depression angle reading at $2.5$ m marking on the staff is $6^{\circ}10’$. The horizontal distance between A and B is $2200$ m. Height of instrument at station A is $1.1$ m and R.L. of A is $880.88$ m. Apply the curvature and refraction correction, and determine the RL of B (in m) _______