As the other posters have mentioned you can over estimate your income, but their is also to components. FTB A which is the combined family income and FTB B which is the in cases of single parents they automatically receive and in couples it depends on the amount the lower income earner receives. Sometimes if the secondary income earner starts working it is very easy to get an overpayment with the Part B component.

You also have options on how you would like to receive your payments you can receive all fortnightly. FTB A a minimum rate and the rest paid when you lodge your Tax Return, you can elect to not receive part B until then end of the fin year.

A good place to look in on the Centrelink Website, the have an FTB Estimator, you came put in different figures and see how it will effect the rate you are entitled to.