Once the ball leaves the stadium, guesswork plays a major role in determining the distance. Each stadium has a grid that shows distances from home plate to various points where home runs could land. If there is a bullpen or seats beyond an outfield fence, finding the spot where the ball lands and figuring the distance is simple.

A team employeeusually someone in the media or public relations officedecides where the ball lands based on what he or she saw and on video replays. The spot where the ball landed corresponds with the grid and a distance is found.

If the ball lands in a lower deck, the distance is found on the grid. If it is in an upper deck, a formula is used.

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The height from ground level to the point where the ball lands a number found on the gridis multiplied by an index assigned to the trajectory:for a line drive, by 1.2; for an average fly ball, by 0.8; and for a high fly ball, by 0.6.

That number is added to the distance of the point that corresponds at ground level, usually in the lowest deck.

Sound complicated? Sort of, but much easier to figure than many Fenway homers.

When the ball clears the net above The Wall, the same formula is used. But it is based on the trajectory of the hit, the distance of The Wall at the point the ball cleared, the height of The Wall (37 feet), and the height of the net (56 feet).

When balls sail across Landsdowne Street, there is more room for error and second-guessing.

Obviously, the balls that clear The Wall are the toughest, said Boston Red Sox public relations assistant Adam Levin, who has been figuring the distances the past season.

When McGwire’s homer landed on the roof of a parking garage last month, Red Sox intern Ken Kams heard questions in the press box about how he determined the distance. His distances, according to some, seemed low.

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All you can do is go by the formula, said Kams, a student at Tufts University. There is judgment involved, but I don’t think we’re ever off by that much.

Some teams have been accused of downgrading home runs by opposing players, while being more liberal with the estimates for their players. The accusation has been made at Fenway.

Problem is, home runs over The Wall are sometimes deceiving. The foul pole in left field is about 309 feetit was changed from the previously posted 315 this seasonand The Wall juts to 386 in left-center.