Excerpt: Mr. Kotlikoff and his colleagues have developed and market a tool called ESPlanner (www.ESPlanner.com) based on economists' theory of 'consumption smoothing' -- the idea of maintaining a relatively level living standard over your lifetime. A consumption-smoothing model focuses on your likely spending and saving needs over time, accounting for temporary big outlays and a changing household structure (for instance, once the kids leave home, your expenses change). It aims to make sure your living standard remains steady, rather than suffering jolts as you try to sock away, say, 20% of your income every year no matter what comes up.

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