(MONEY Magazine) – You can mail five sheets of paper with one 25 cents stamp. The spaghetti is ready if it sticks to the wall. You will get 80% of the value of this article in the first 20% of the time you spend reading it.

Rules of thumb are the unseen umpires of our daily lives. They give us certainty when life is ambiguous. Without them, surely our decisions would be infinitely more confusing. Nowhere does folk wisdom sprout more thickly than in the field of our finances. Hear now the voices of experience: ''Do not refinance your mortgage unless the new rate is at least two percentage points below the old one. . . Budget 25% of your salary for shelter. . . You will need 75% of your current income to retire comfortably.'' Those maxims and dozens like them are the fast food of knowledge: convenient and unchanging. But reliable? Sometimes. ''A rule telling you to set aside 10% of your take-home pay is useful only in that it conditions you to save money,'' says Lawrence A. Krause, a financial planner in San Francisco. ''But 10% is simply a number people can relate to; it could just as easily be 5% or 15%.'' Which truisms really make sense? Which deserve thumbs down? Following are some of the rules we live by: -- Spend no more than 25% of your take-home pay on housing. When property prices shot through the roof in the hyperinflationary late '70s and early '80s, this old saw went the way of love beads. If you bought a house last year in a major metropolis, your mortgage payments more likely eat up a full third of your after-tax income. Of course, the enormous diversity in housing prices across the country -- from the boom in California to the doom in oil- patch states -- puts any generalization on shaky ground. -- Buy life insurance with a death benefit equal to five times your annual income. Useful as little more than a sales tool for insurance agents, this credo could easily leave you underinsured or overprotected. How much coverage you really need depends on the kind of household you live in and your other assets and liabilities. If you are single, or married with two incomes but no kids, for instance, you may not need it at all, and you certainly will need far less insurance than a family with college-bound children and a hefty mortgage. Similarly, if you have a substantial investment portfolio that your family could tap if necessary, you can get by with a smaller policy than a household with few financial assets. -- Set aside emergency cash reserves equal to three to six months' worth of your after-tax income. No one would argue against the wisdom of building a savings cushion to tide you over a short-term cash crunch. But the particulars of this guideline need fine-tuning. ''It makes more sense to set aside living expenses than income, since that's what you will need to replace,'' suggests Steven Enright, director of financial planning at Seidman Financial Services in New York City. Precisely how big your emergency fund should be depends on your family situation and your job. For example, dual-income families may be able to make do with a cache equal to just one or two months' expenses, but households dependent on a single salary need more. If you have a steady, salaried job, three months is enough, but if you are self-employed, and the income from your business is precarious, shoot for six months. -- Do not refinance your mortgage unless the new rate is at least two percentage points lower than the old one. This guideline neglects a crucial factor in the refinancing equation: how long you plan to stay in your home. The sooner you intend to move, the greater the rate differential must be between your old and your new loans for refinancing to make sense. Reason: you will probably pay from $3,000 to $6,000 in points and closing fees on a new loan. If the rate on your new mortgage is two percentage points lower than your old one, it will take three to four years before savings from lower monthly payments offset the up-front charges of refinancing -- a good deal only if you plan to stay put for more than four years. -- You should earn a salary equal to your age in thousands. While it is nice to think of Donald Trump as a million years old, this adage has no grounding in reality. The size of your paycheck reflects the field you are in, the position you hold within it and the credentials you bring to the job, not your age. In fact, top professionals earn far more than their age in thousands throughout their careers, according to a recent salary survey by Johnson Smith & Knisely, an executive search firm in New York. Their findings suggest a new twist on the old rule of thumb for ambitious careerists: you should be earning at least twice your age in thousands by the time you hit 30 and roughly three times your age by the time you turn 40.

-- Your credit-card balances and other consumer loans should total no more than 20% of your after-tax income. In our credit-addicted nation, this salutory maxim should be regarded as a golden rule -- particularly now that interest on consumer debt is only 20% deductible. -- If your car is three to five years old, drop collision insurance. Unless you own a Rolls, this guideline makes sense. Why should you shell out several hundred dollars a year to protect yourself against damage to a car that has depreciated in value to a few thousand dollars or less? -- Dual-income couples should set aside one spouse's salary for savings. If you can afford to stash away such a hefty sum, by all means do so. But the underlying assumption that only one breadwinner's paycheck is necessary and any other income is gravy is a notion that went out of fashion when the Cleavers left prime time.

-- If you plan to move in three to five years, rent instead of buying a home. Depending on where you live, you will probably need at least this long to recoup the extra costs of home ownership, including closing fees, insurance and property taxes. -- If you trade in your car every four years, you are better off leasing. If you change cars as often as Meryl Streep changes accents, this rule bears consideration. A new car depreciates fastest in the first few years of ownership. The outstanding balance on your car loan may be greater than the money you'd make by selling your car if you trade it in sooner than four years. -- To retire comfortably, you will need 75% of your current income. Use this ball-park figure only if you're an all-star who has paid off your mortgage and whose employer will continue to provide health insurance after you stop working. If not, you could need more. (For more on estimating retirement costs, see page 195.) -- Keep 5% to 10% of your portfolio in gold. ''This is a great rule for the precious-metals industry,'' says John Markese, director of research at the American Association of Individual Investors in Chicago. Although gold has historically done well in high-inflation periods, Markese notes, most people get adequate protection from rising prices through home ownership or other real estate investments. -- Make the maximum contribution to your 401(k) every year. Sound advice but not necessarily your most important priority if you are saving for a down payment on a house or your kids' college tuition. While stashing the maximum $7,627 in your company savings plan will reduce your taxable income while allowing your investment earnings to grow tax-free, you usually cannot withdraw your funds without stiff penalties until you are at least age 55 and have left the firm. -- Rule of 72. At last, a rule of thumb that's always right! Want to calculate roughly how long it will take you to double a sum of money at any given interest rate? Simply divide 72 by the anticipated annual rate of return to get your answer. So an investment earning 6% should double in about 12 years, and one earning 12% will double in around six. That is, if you don't take any wooden nickels.