Ticking the box will display seam level contours and clicking on one of those green lines will give depth relative to OD in metres.
Just make sure that you use the contours for the same seam, given as a seam code number, to work out the dips - there are several different seams shown on these composite plans. The workings closer to the shafts are fairly certain to be those of the relevant pit - further away they may be over/under working from one of the adjacent pits
It is a bit puzzling at first but it does get easier when you've played about with it a bit.