What's my retirement 'Number'?

NEW YORK (CNNMoney) -- What number do we need to hit to retire comfortably? $1 million, $2 million, more? -- Jim Rodgers, Madison, Wisc.

Your question is timely: Mutual fund titan Fidelity recently released its estimate of how much people should have in savings to retire. The Number: eight times final salary.

That figure caught my attention because it's a lot lower than most targets you see. Just last month MONEY recommended you shoot for a nest egg of 12 times your income by retirement.

These numbers vary for many reasons, which I'll get into below. But the real issue here is that no single figure can possibly reflect your specific financial circumstances. At best this kind of number is a ballpark estimate for how much you should save, not a substitute for planning. Before you use one as even a rough guide, though, you should know what goes into it.

Fidelity assumes retirement at age 67 and figures your savings must last 25 years. We reckon you'll retire at 65 and want your savings to support you for 30 years. Those two more years on the job, which can boost your Social Security check by 15% to 20%, plus the fact that your savings don't have to last as long, shrinks your target.

Related: Why high-income savers need to put more away now

Another variable is how much you'll draw from your savings. Fidelity aims to provide 85% of your after-tax income, an approximation of what you were actually spending. After estimating how much Social Security will provide, the number crunchers calculate your withdrawals. With a $75,000 salary at retirement, you'd need to take just over $35,000 from your savings the first year.

This methodology seems reasonable to me, but then things get tricky. Pulling $35,000 from a $600,000 portfolio ($75,000 times eight) represents a withdrawal rate of almost 6%. To have a reasonable chance of your savings lasting 25 to 30 years, most advisers suggest something closer to 4%.

When I asked Fidelity senior vice president Steve Feinschreiber for the probability that a portfolio would last 25 years starting with a near-6% withdrawal rate, he estimated it "very roughly" at 70%. A Morningstar retirement-income calculator put the likelihood at about 50%.

Related: Can I afford to retire early?

Finally, the higher your income, the less of your salary Social Security will replace and the more you'll have to withdraw. In fact, Fidelity concedes that with a six-figure income your target might have to be higher than eight.

To be safe, I'd recommend you shoot to save 10 to 12 times your salary. Better yet, use an online tool like Fidelity's own Retirement Income Planner or T. Rowe Price's Retirement Income Calculator to get an estimate of your retirement readiness.