According to this, you have two very good rules of thumb or models you can use to determine this:
1) The heuristic bi-linear model. We made this by making the best bi-linear model a bit simpler to apply.
It goes:
If you’re under 85, your life expectancy is 72 minus 80% of your age.
Otherwise it’s 22 minus 20% of your age
2) The 50-15-5 model. This one asks you to remember some key values and then to interpolate between those values. It goes:
The life expectancies of 30, 70, 90 and 110 year olds are about 50, 15, 5, and 0.
For more, check out the link.
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