"The figures are staggering . . . beyond our wildest dreams."

That's the developer's assessment of White Flint, the sophisticated shopping mall on Rockville Pike that opened nearly a year ago as a $50 million gamble.

Despite all the whispered words of doubt by some area retailers, that the first thing Washington needed was another affluent shopping center in Montgomery County, White Flint developer Theodore N. Lerner was able to quash the second-guessers by revealing a rate of initial sales growth that has surpassed even Tysons Corner Center, a Lerner Corp. creating that is one of the nation's top retail Meccas.

For example, the "staggering" figure cited by Lerner is the sales volume during the first nine months of operation that is 27 percent ahead of projections. That means that annual volume of White Flint will range between $125 million and $150 million in its first year - perhaps a record for few malls anywhere.

Moreover, White flint is about to set a third major store, Lerner confirmed in an interview and walking tour at the mall this week.

I. Magnin, a California-based specialty chain, will open its first store cast of Chicago at White Flint next September - joining Lord & Taylor and bloomingdale's as the center's major tenants.

Magnin has leased 80,000 square feet in the mall for a two-floor store that will include a glass-enclosed central escalator well with store entrances from several points with the mall.

Although owned by Federated Department Stores, the same firm that owns Bloomingdale's, Magnin has a reputation for more costly thorough lines in special areas not matched by other retailers - including china and women's wear.

Magnin's publicity people had hoped to holf off the announcement of their Washington addition until March, but the firm's spring catalogue already is in the malls - featuring dozens of pages of high fashion and a one-page insert that states:

"I. Magnin is looking towards Washington . . . and the discerning people who live there . . . Your name has been suggested as an I. Magnin charter charge account customer for our new store which will open in White Flint in Early fall . . ."

Perhaps more important for White Flint is the news that Magnin will occupy available space (some of which had been planned for offices) and will not replace an existing retailer, as had been rumored.

Lerner himself took a direct approach recently when he heard rumblings that Lord & Taylor, the New York specialty store owned by Associated Dry Goods Corp., wanted to pull out of White Flint. He wrote a letter to seek information on the store's success to date.

The rumors were absolutely not true, replied Lord & Taylor officials, who said the White Flint store not only was adding up sales far above expectations but already had become one of the best performers in the entire chain.

According to Arthur F. O'Day, vice president of Associated, " . . . In its first year of operation (the White Flint Lord & Taylor) will be one of the ten most productive stores in volume per square foot in the Associated Dry Goods Corp. operations. We will finish this year with 146 department stores, so you can easily see that White Flint's Lord & Taylor store is an unqualified winner."

Bloomingdale's monthly sales volume at White Flint, meantime, has been exceeding that of its first store at Tysons Corner. Both Bloomingdale's and Lord & Taylor are seeking additional sites for stores in this area. Magnin is expected to build a second store, too.

If the major stores are doing well, 90 other shops in the 800,000-square-foot mall are doing better. Sales are averaging $200 a square foot of retail space a year (unusually high in retailing) compared with a projection of $125, Lerner said. Bloomingdale's volume is about $150 a square foot compared with $75 for many department stores; Lord & Taylor exceeds $100 a square foot.

The most unusual development in the short history of White Flint is not its higher-than-projected sales, however. Its special feature, Lerner said, is that for the first time in the metro politan area, a retail mail is drawing customers from the entire region - Northern Virgina, the District and the Maryland suburbs. It is not a local mall, drawing most of its business from a nearby marketplace.

In addition, bus loads of shoppers arrive from Baltimore and conventions in Washington have added a White Flint "day" for spouses.

The key to White Flint's success, Lerner said, has been the selection of a variety of merchants sho provide a mixture of price ranges. Many of the traditional departments of a major department store are dotted throughout the mall, behind unusual facades, but under family or chain ownership that provides more individual service.

By fall, the mall will be 80 percent leased compared with a third for the first year at Tysons Corner. A new Lewis Thos, Saltz store will open next fall, along with Magnin and a section of shops called "M Street," a replica of a cobblestoned street in Georgetown.

A 21,000-square-foot "Via Rialto already is open, providing space for small shops within the center. A separate market square has been organized with butcher, baker and a forthcoming produce stand. There's also a financial wing, with branches of Citizens Bank & Trust, Government Services Savings & Loan and American Express (to open soon) and a stock brokerage.

"The Eatcry" is White Flint's answer to fast food service, featuring a dozen international cuisines and serving 80,000 meals a week. On the mall's third level, a series of sit-down restaurants also are being opened (one will be a disco).