Special Longevity Calculation

In a general reading we attempt to find the duration of life for the practical reason of knowing when we should end the reading. We simply try to estimate the longevity for this practical reason. If somebody wishes to have a much more exact determination of the longevity it can be done. For this purpose we go to a lot more trouble and try to get the time to within a few months depending how far into the future we have to look. In past case we have been accurate to within a few weeks predicting from about 3 years before the event took place.