Jack Rankin Jr.'s day job is at Honeywell Federal Systems in McLean, where he is a communications support group manager. But for fun, he gets out a crystal ball and tries to guess how much a Big Mac will cost 10 years from now, and at this he is pretty good.

The Round Hill, Va., resident won $10,000 in a Forbes magazine contest for the Big Mac forecast and other nearly correct predictions -- including the prime rate's level and the number of pro football teams -- he made 10 years ago. Rankin out-predicted 660 readers who entered the contest announced in the Oct. 27, 1980, issue of Forbes, the magazine said.

Under the point system Forbes established for grading the entries, based on nearness to the actual figure, the 59-year-old Rankin had one perfect score, for his exact estimate of the number of pro football teams (unchanged from 1980), and earned points in 22 of the 34 other categories. He also was close to the mark for the price of a Big Mac -- he forecast $1.60, while Forbes said it was $1.79 at the end of last year.

No one's perfect, of course. Rankin forecast the Dow Jones industrial average would be 1150; it was 2753 on Dec. 31. He said the price of a barrel of Saudi crude oil would be $59; it was $18.40. A few hundred energy analysts missed that one, too.

Malcolm Steve Forbes Jr., editor-in-chief of the magazine, announced another contest for the next decade in the magazine's June 11 issue. This time the pot's up to $100,000. Forbes's advice to entrants: Don't assume today's trends will continue.