Same Shambini, we thought we had a pretty generous provision but still managed to go over!
We also have had some other variations which we didn't allow for - sarking , fly screens, we also upgraded the insulation to match one of designer homes. I understand from one of the other homeone members that where we get sarking, we need whirlybirds. All of this has added up.
May be that is why Metricon ask you to do the finances after colours etc. Which we didn't want, as we knew we will have to argue with the valuer - it took around 3 weeks of back and forth to get them to up the val - which we still think is incorrect, but we can live with it.