Steve, MFD = MultiFunction Display. He's asking if he should use manual calculations or trust the car
In reality, the database is not 100% accurate because there are different ways of calculating averages and cars also have different amounts of error. It is by far easier to use the MFD's reading, as I do. However, if you prefer to do it manually, by all means. If this was a pure Prius community, I would suggest either one or the other, but as GreenHybrid wecomes all hybrid owners as well as other interested individuals, there is little hope we can all come to common grounds.
In short, whatever you want. In the long run, it won't make a huge difference anyway.