To calculate a restaurant server's tip, double the first number on the bill. A $62.47 tab gets a $12 tip. If the bill is more than $100, double the first two digits.

That's an example of a money rule of thumb that is imperfect but useful, which is the idea — inexact starting points for goals, conversations and calculations.

Americans apparently could use the help. Many are winging it through their financial lives without confidence in their ability to afford retirement, an emergency expense or even daily living costs, according to a survey by The Associated Press-NORC Center for Public Affairs Research.

Money-related benchmarks can help and are especially timely as we turn to a fresh decade and make money resolutions anew. Still, finances need regular monitoring for budgeting, assessing progress toward goals and evaluating debt reduction, said Paul Golden, spokesman for the National Endowment for Financial Education.

"As you condition yourself, you can build on more time (to review finances), but use a half-hour a week as a starting point."

Some money rules are broad:

— Try to save 15% of income for retirement. Aim to replace about 70% of your preretirement income. And when you tap the nest egg, drain just 4% per year.

— Start an emergency fund with $500 and eventually build it to three to six months of essential living expenses.

Other rules address specific situations:

— Limit student loan borrowing to the amount you expect to earn in your first year working.

— Full-time hourly workers can double their wage rate and tack on three zeroes to approximate their annual earnings. Making $15 per hour yields about $30,000 per year.

Professional advice and online calculators will provide more accurate and detailed answers, especially for people who have unusual money situations. But here are some handy rules of thumb to get started.

SPENDING

— 50/30/20 BUDGET. Figure half of your take-home pay should go toward "needs," such as housing, food and transportation. Then 30% goes to wants, and 20% funnels to savings and debt repayment.

— RULE OF 10. For big discretionary purchases, reflect on how it will make you feel in 10 days, 10 weeks and 10 years. Perspective can calm buying urges for purchases you later regret. Related: Give yourself cooling-off time equal to one day for every $100 the purchase costs.

— TERM LIFE INSURANCE. Buy a policy worth 10 times your gross annual income only if somebody else depends on your income.

— KID ALLOWANCE. Give $3 weekly per grade level in school. A fourth-grader gets $12. The overall average is $30 per week, according to a survey by the American Institute of CPAs.

— WINDFALL. Do responsible things with cash infusions, like a tax return or inheritance. But set aside 2% to blow on something fun, so you don't feel deprived.

HOUSING

— HOUSE PAYMENT. Your mortgage, including taxes and insurance, should not exceed 30% of your gross monthly income.

CARS

— CAR PAYMENT. Limit payments to 10% of your monthly take-home pay, so you can keep your total car costs — gas, insurance, repairs and maintenance — below 20% of your income. Also, put 20% down and limit the loan term to four years.

— REPAIR OR REPLACE. Replace your car if a repair costs more than your car is worth — as determined by, say, Kelley Blue Book — or exceeds one year's worth of monthly payments.

SAVING AND INVESTING

— NET WORTH. Net worth is the number that sums up your money life. One measuring stick: All you own minus all you owe should equal your age times your gross income divided by 10, according to the book, "The Millionaire Next Door."

— RULE OF 72. Divide 72 by your expected annual rate of return to estimate how many years it will take for an initial investment to double. At 6%, the investment replicates in 12 years.

— FINANCIAL FREEDOM. Achieved when savings are at least 25 times your annual expenses.

CREDIT AND DEBT

— TOTAL DEBT. All debt payments, including mortgage, should be less than 36% of monthly gross income.

— CREDIT CARD BONUS. On rewards credit cards with an annual fee, look for a sign-up bonus value equal to three years or more of its annual fee unless it has especially valuable rewards or benefits.

— CARD USE. Keep credit card balances at 30% or less of their limits to avoid hurting your credit scores.

This column was provided to The Associated Press by the personal finance website NerdWallet. Gregory Karp is a writer at NerdWallet. Email: gkarp@nerdwallet.com. Twitter: @spendingsmart.

RELATED LINKS:

NerdWallet: Budgeting 101: How to create a budget:

http://bit.ly/nerdwallet-budgeting-101

AP-NORC Poll: Many feeling vulnerable despite economic gains

http://bit.ly/AP-NORC-poll

Kelley Blue Book: What's my car worth?

https://www.kbb.com/whats-my-car-worth/