When you are estimating sums and differences of fractions by rounding how do you know to round it to zero half or one?

Let's say that you are rounding 2/5. You find half of 5 which is 2.5. Then you see if 2 is closer to 0, 2.5/5, or 1. It is closer to 2.5, so you would say that 2.5 is half of five, so your answer is 1/2. If you are doing 1/7, 1 is obviously closer to 0, so your answer would be 0.