Well, lets say we have to estimate 2/4. Since 2/4 is half of 4, it stays the same. Now let's say we also have to estimate 7/12. Since 7 is higher that 6(which is 1/2), 7/12 should be 1 whole. So technically, if it's higher than half of the denominator(6 is the half & 12 is denominator.) it should be 1 whole but if it's lower, then it should be simply 0.