Procedure:
1. Set up the theodolite at Q and level it accurately.
2. With both plate clamped, the vernier A reading 360o take back sight on P (i.e bisect ranging rod at exactly)
3. Transit the telescope to direct the line of sight produced by PQ.
4. Loose the upper plate and turn to telescope clockwise to take fore sight on R (i.e. bisect ranging rod at R exactly using upper clamp and its tangent screw). The mean of two vernier readings give the approximate value of deflection angle at Q.
5. Loose the lower clamp and turn the telescope horizontally to back sight on P. The verniers will read the same reading as in above step and the telescope inverted.
6. Transit the telescope. Unclamp the plate and again bisect R read both verniers.
7. Find the mean of final vernier reading. Thus the deflection angle is doubled and hence, one half of this average value gives the accurate value of deflection angle at Q.