I don't want to label myself an expert on it, but I have flown a few T&B's on the 412.
Be very precise with the ground track and then the hover track. With those two regimes done well, the rest should follow. Then focus on the forward flight. Try to get all regimes on each flight. Also, remember to compare the results of each flight to make sure you're not going back to an earlier condition.
It can take a long time, but it does give you a smooth ride that tends to stay smooth for many hours.
Matthew.