Here's a practical, four-step approach to sane decision-making in today's real estate market, courtesy of 10 of the nations top banking and mortgage economists.

* Put together a "buy-sell equation" for yourself before deciding to do anything--sell, buy or both--in the next six months to a year.

Count on conventional mortgage rates being no lower than 15 percent before 1983.

* Count on the average price appreciation for houses becoming no greater than the overall rate of inflation in the economy--probably in the single digits.

* Don't be numbed by today's real estate financing and price conditions. Get used to the sound of them. They're here for the long run, certainly through the mid-1980s.

That's the consensus advice that emerges from discussions with a blue-ribbon, informal panel of economists who specialize in housing and finance. None of them claims to have an absolute lock on where the U.S. economy will be a year from today. None of them owns a crystal ball. But all of them make their daily bread and butter by advising large, multimillion-dollar financial institutions or trade associations. And all of them are homeowners, with a very practical stake in the real estate field.

The rationale for a "buy-sell equation" was explained this way by Robert Parry, senior vice president and chief economist for Los Angeles' Security Pacific National Bank, the 10th largest bank in the country:

"More than ever before, sellers or buyers need to lay out the true, economic costs of housing decisions on both sides of the transaction, in advance of making them. They've got to look coldly and analytically at what financing discounts do to true selling prices, at leverage, and at taxes--and to forget their old preconceptions. They may be surprised at what they find."

Without a rough equation, Parry said, "people are apt to either stay out of the market unnecessarily (because interest rates seem scary), or to go chasing after 'bargains' that are actually very dumb moves."

Carroll Melton, economist for the Chicago-based U.S. League of Savings Associations, described how his own "buy-sell equation" works for a prospective seller.

First, the seller needs to calculate the true, discounted selling value of the property involved. For example, if the owner of a $90,000 suburban home with no assumable financing finds that the only way he can sell this fall is by offering $50,000 worth of financing assistance to purchasers at 12 percent for three years, "the seller has to face economic reality about the true price he's getting."

"He's not getting $90,000 for the house," Melton points out. "He's going to get $90,000, minus the discounted value of the mortgage financing he's providing. If the seller invested his $50,000 at 17 percent for the three years--instead of getting 12 percent on it for three years from his buyer--he'd have 15 percent more money at the end of the 36 months (5 percent difference times three years.)"

The true price the seller is going to end up with is something on the order of $82,500--a figure that may be entirely acceptable. However, with rates likely to moderate over the coming year, Melton adds, the seller ought to know that he might get a higher true price by selling in late 1982 (when mortgage rates should be 15 percent, and discounted seller-financing proportionately less), than he'd get today.

The other side of the seller's equation, notes Melton, concerns the house or condominium he has the opportunity to purchase. How does it stack up to the discounted, true price? Is the cash value of its attractive, deeply discounted seller financing actually tacked on to the asking price--making it no bargain at all? Does the low down-payment financing permit the new buyer to improve his own leverage position (that is, owning more house but investing less cash)?

"If the seller is going to get a price from the sale of his house that's 10 percent less than he really wanted (because of the cost of financing), and he's going to pick up a new condo or house that is 20 percent to 30 percent below the market price of a year ago--then the equation may well be saying 'buy'."

Using a personalized buy-sell equation, according to Ray Broughton, vice president of Portland (Oregon's) First Interstate Bank, "can put you into surprisingly smart investments in the coming year, despite what appears to be a horrible economic climate to do anything in real estate."

Or to put it another way: Sharp consumers can turn up true bargains--and avoid bad moves on sales and purchases--by looking extra hard at the numbers on both sides of the equation. Given the high-rate market we are in, you just may find the scale is tipping your way.