The “rules” of finance — how much to pay in taxes, what to contribute to retirement — might often seem surprisingly vague.

Here’s another potential surprise: Most financial planners have a similar take on these “rules of thumb.” Financial advice meant for the “average” person is usually just that: average. It’s not good enough to get you that worry-free retirement, the dream of a small business, or money for kids’ education.

That said, I will concede that there are a few general rules that can provide useful direction, even if they are not sufficient to get consumers to their ultimate destinations.  At the same time, there are other suggested rules of thumb that, in my opinion, have too many exceptions to be very helpful.

RULE OF THUMB:  Subtract your age from 100 to get the percentage of stocks you should hold in your portfolio. The rest should be in fixed income. 

THUMBS DOWN! This too-simple rule assumes all individuals of a given age are financially identical. Your portfolio allocation needs, at very least, to also take into account the size of the portfolio. An 80-year-old millionaire probably does not need to hold 80 percent in low risk bonds, when those assets will likely never be used by him, but rather by his younger heirs. Those who have ample retirement pensions may also be exempt from this rule, even if their portfolios are far more modest.

RULE OF THUMB: A good plan for retirement is to have seven to 10 years of annual living expenses always available in cash, short-term bonds and/or guaranteed income.  

THUMBS UP!  This rule ensures a retiree has ample liquidity to get through a down period in the financial markets without having to sell stock holdings. It also gets to the heart of the retirement issue: namely, that retirement success depends on the level of your living expenses.

RULE OF THUMB: You’ll need 80 percent of your earnings for annual expenses in retirement.

THUMBS DOWN! This old rule needs retiring. Today, many people retire to an active lifestyle. They travel, go back to school, remodel their homes, and therefore spend more than when they were working. Then there are escalating medical costs, combined with the likelihood that we’ll be paying more of those costs out-of-pocket. Throw out the rulebook in this instance, and instead do a bottom-up analysis of your actual retirement expenses.

RULE OF THUMB: You can afford a house worth 2.5 to 3 times your annual income.

THUMBS DOWN! This rule falls short in many ways.There are professionals with seven-figure incomes who cannot afford a $500,000 home, and there are couples just starting out who thrive on the financial discipline of stretching to pay for a home that is four or more times their income. Affordability is not necessarily a function of income, but rather what people do with that income.

RULE OF THUMB: Your total debt should be 36 percent or less of your gross income.

THUMBS UP! This is a better “affordability” rule than the previous one, and is often used by banks to determine how much of a mortgage — and thus how much house — you can get. It also takes into account  that two people with the same income can look very different financially, depending on their liabilities.

RULE OF THUMB: Financial success requires living within your means. 

TWO THUMBS UP! This is a true golden rule of personal finance and comes as close to being a universally applicable rule of thumb than any other. Even I, with my professional training, resort to this rule to give me an instant read on an individual’s ability to meet financial goals. If I see someone who invariably consumes less than they make, it’s almost a sure thing that they will be financially secure, regardless of their level of income or size of their portfolio.