The dreaded manila envelope came from the tax man last week. But (sigh of relief) it was only to announce that my long division was faulty and the glorious state of Maryland was sending an extra $15. This news is passed along as cautionary information because today the faulty mathematician puts a pencil to the economics of Patrick Ewing's future.

Should Ewing stay at Georgetown University? Under the new NBA contract, with its limitations on payrolls, would he lose millions of dollars if he didn't turn pro now? Or does the free-agent clause of that contract guarantee him top dollar if he waits until it takes effect?

We're talking only economics. Ewing is happy at Georgetown, content with the education and discipline he sought. He would become a better player under John Thompson's coaching two more seasons. Those years at Georgetown also would help Ewing, as they helped Ralph Sampson at Virginia and would help any student, grow up socially.

So money is only one factor Ewing might consider before the May 14 deadline for turning pro. He has said he wants to stay at Georgetown. But that was before the full implications of the NBA contract were clear. Thompson has advised him now to gather information before making any decision.

Ewing ought to gather some aspirin, too, if the process gives him the headache it gave me trying to figure it out. These are my inconclusive conclusions: 1) Ewing could lose millions of dollars, over the length of his career, by staying in school; 2) he could gain millions by staying in school, and 3) he's going to get rich either way.

Let's say you're Patrick Ewing.

You know the NBA payroll limit, effective for the 1984-85 season, is projected at $3.6 million per team (based on 53 percent of league revenue).

The minimum is projected at $3.2 million.

Five teams reportedly are over the maximum: New York, New Jersey, Los Angeles, Seattle and Philadelphia. Those teams' limits were frozen at their current level. Eight teams are under the minimum, including the Bullets at about $2 million.

The NBA contract also says first-round draft picks can choose to sign a one-year, $75,000 deal that gives them free agency after the rookie season. Also, teams over the $3.6 million ceiling cannot--repeat, not--sign a rookie for more than $75,000, without making other roster moves. The team's payroll maximum is the only other limit on a rookie's salary.

Owners and players praise this collective bargaining agreement as the NBA's salvation. Larry Fleisher, the players' union boss, says no player will be hurt. Larry O'Brien, the NBA commissioner, calls it a landmark in pro sports.

Here's the situation for Ewing (and Akeem Abdul Olajuwan, Sam Perkins, et al) . . .

If Ewing turned pro this spring, he likely would be the second player chosen in the draft.

Either Houston or Indiana would pay big money for a franchise-making player. Using Moses Malone as a measuring stick at $2.2 million a year, Ewing ought to be worth $1 million a year right now.

But what if Ewing waits until graduation? What will his financial picture be when the NBA's "landmark" contract is in effect?

It is likely that those lowly teams, with low payrolls, still would be willing and able to pay Ewing $1 million without exceeding the $3.6 million maximum.

It is possible that those teams, if they're up against the payroll limit, could get rid of a high-priced player to make room for Ewing's contract. Heaven knows there is a forest full of expensive deadwood on NBA benches.

The chilling possibility exists, however, that one of the teams over the maximum will own an early first-round draft choice. If Ewing were drafted by such a rich team, it could sign him only to a $75,000 contract--unless it cut its payroll, gaining room to pay Ewing the amount by which the team is under its limit.

A perplexing question: would a rich team, successful on the court, get rid of established players to give Ewing $1 million?

Another thing: if Ewing were to sign a $75,000 deal two years from now, the free agency aspect would not be the lever of power it seems. That's because he wouldn't be a free agent until the second season of this collective bargaining deal. By then, it seems likely, most teams would have raised their payrolls near the maximum and effectively would be out of the big-bucks free-agent market.

Ewing, under another provision of the new deal, then could re-sign with his original team at any figure (even if it put the team over its limit). That's good. The bad part is his leverage would be diminished then by the lack of other bidders, and he might have to take his club's offer.

Employers, being mostly human and thus susceptible to human nature as well as good business sense, love to get the most work at the cheapest cost. However generous the club would claim to be, Ewing's second-season salary in such an example likely would be less than he would have brought in the open market of Moses Malone's day.

So it is possible Ewing would be forced to sign for $75,000 and then forced to accept a second-year deal worth, say, $500,000 a year.

The NBA owners have conspired (albeit with consent of the players, frightened that the league might collapse) to create a bizarre sanity. But who knows how long sanity will prevail?

Maybe Ewing would be so sensational a rookie that the other 23 teams would do anything to sign him up. Trade Larry Bird for a $75,000 rookie, make Magic a movie star, dump the aging Dr. J. Then some crazy might give Ewing a $2 million deal.

Such a possibility muddies the water even more, leaving only one thing clear: Ewing's decision won't be easy if economics is a major factor.

Pass the aspirin, please.