Clearly, a household's need to estimate their average (yearly) income is paramount; spending, which will be relatively constant, needs to be maintained below that amount. A budget being an approximate estimation, room for error should always be allowed so keeping expenses 5% or 10% below the estimated income is a prudent approach. When done correctly, households should end any given year with about 5% of their income left over. Of course, the better the estimates, the better the results will be.