PG, my 'open pit' thoughts without a picture to support it,...

PG, my 'open pit' thoughts without a picture to support it, although it would probably help, a lot.
Assuming; DOF is at a 35degree angle so you could imagine using the 'deep low side' of it as one of the pit walls. The far wall of your pit (triangle) could be at 55degrees (to keep things simpler, 90 degree angle at bottom).
Let's also say DOF is about 4m thick.

To extract the first 100m 'in ground' length of DOF, you'd have 100m x 4m=400m3 (cubed) of DOF material (no I'm not doing 2.8 density, just size). The waste off to the side would be about 3,500m3.
True depth vertically would be roughly 57m.

If you wanted the next 100m length (400m3 of DOF), you'd have to move another 3,500m3 of waste + another 7000m3, (10,500m3 waste).
True Depth would now be roughly 114m.

If you wanted the next 100m length, you'd have the previous waste again, plus another 7,000m cubed (17,500m3).
Every extra 400m3 of DOF material would include an extra 7000m3 of waste (on top of the previous amount).

Thus, forget open pit for depth.