The half-life is about 0.7 of the mean life; the mean life is about 1.44 half-lives.
The mean life is 1/λ and the half-life ln 2/λ, so their ratio is ln 2 = 0.693. The average atom lasts longer than the half-life because the few that survive a long time pull the average up: after one mean life, 37% are still there, not half.
The half-life describes the sample and the mean life describes the atom; confusing them makes lifetimes look 44% too short. The mean life says nothing about when a particular atom will go: because the decay is memoryless, an atom that has already survived a long time has exactly the same chance of going in the next second as a new one.
Estimate with the rule, then check it against the calculator that models it properly.
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