From an instrument position, a BS reading of 1.8m is taken to nearby benchmark(BM) of RL 165m. FS of 3.5m is taken to station P at a distance u of 2.6km. now the instrument is placed at the station P and the BS observed will be 0.8m. if FS of 2.6m is taken on an inverted staff placed on the ceiling of a room then calculate the exact RL of the ceiling.