So, if total volume 1 m3, number of bricks required = 1/0.00244 = 409.83 = 410 (say). So, the volume of mortar = (0.00244 - 0.00189)*410 = 0.2255 m3. Now, if we consider 1:4 ratio of mortar then, Cement required, = 0.2255*1 /5 = 0.0451 m3.