If you feel the need to reduce the auto-tip, go to the purser's desk. They will usually want to know exactly what the problem was and why you did not complain earlier, so that they could fix things. A valid question, I suppose, but the one time I did this, the true answer, which I gave, was that we kept trying to ignore it and keep a good attitude. They asked for my cruise card and did the transaction on the spot. They also asked if I wanted to speak to the department manager, but this was the last morning and I had no desire to end the cruise with any more negativity than needed.