Rough rule of thumb is anything less than 30 percent remaining needs changing, you can run them much lower but Ive found the heat cannot then dissapate and the tappet boot gaiters then split, so the money you saved on getting another 20k on the existing pads is lost with the cost of the new gaiters.
Most heavy pads have 20mm lining thickness when new, so spprox 7mm is the change time, thats lining only not including the backplate.
100k is pretty low but would depend on the work, where I used to work we saw a lot of merc rigids that carried workwear and they liked rear pads a lot.
On the upside the wiring is a lot better than the Dafs 
If the problem is still there you need to speak to the guy in charge, you will be surprised just how much they will move.
I would guess that pad in your hand has approx 14/15mm lining left?