I don't fly the Dugong but...
I assume by "+ 20 kts" you mean miles? If so, assuming the A380 flies like most other aeroplanes, the slower you descend the further out you need to start. Similarly, I understand Green Dot is best Lift/Drag? In that case, Green dot descent point would be even further out. Lower speed = Less drag-> better glide range.
That is, based on your figures, 300 should be 3x+5nm, 250 should be 3x+10nm. Green Dot should be +20nm.
What's your Vnav tell you?