To correct orthorectification issues, select calibrate above, then click on a 4-way tile join (cross). Repeat for each of the 4 joins.

When in calibration mode, the screen background will go green to alert you. After you click, the calibration error will be (re)calculated and shown, and the background will return to grey. You will need to click the calibration button again before entering the next tile join.
It is not advisable to try to calibrate across map sheet joins, as different sheets tend to have different errors.