The observations of a reciprocal levelling operation carried out from two stations A and B are given in the table. It is known that the instrument has collimation error. The correct staff reading over station B when the instrument is set very near to station A is ______ m (Rounded off to three decimal places). Assume all other errors are negligible.
| Instrument set very near to station | Staff reading at A (m) | Staff reading at B (m) |
|---|
| A | 1.275 | 2.005 |
| B | 1.040 | 1.660 |