Jonny Geller has his cover story down.

On the surface he’s a British literary agent, living a proper middle class Jewish life in London with his wife and two sons. But he secretly runs a clandestine think tank — the Judological Institute of Spiritual Mathematics. Its mission: To boldly go forth and calculate whether something — anything — is GFTJ (good for the Jews).

His efforts brought him to town last weekend to promote his new book, “Yes, But is it Good for the Jews?,” a comical schematic of contemporary society and its impact on the tribe.

He appeared as a guest speaker at the San Francisco Jewish BookFest, held Sunday, Nov. 5, at the Jewish Community Center of San Francisco.

Geller explained to the several dozen attendees in the audience his formula for determining whether or not something is good for the Jews.

Be warned: Math is involved.

“It’s all based on the number 7,” he said. “This number has a lot of meaning to the Jewish people: seven days of creation; the seventh month is the holiest month of the year; Moses wore size 7 sandals.”

First, calculate the anti-Semitic backlash potential of the item in question. Add a value for its impact on the world multiplied by the “J-factor” (i.e., its link to Jewish culture). This gives us the “Tsuris” value, which is then divided by seven. If the result equals 0 to 7, the thing is bad for the Jews. Between 7 and 14: good for the Jews.

Geller ran through a few examples, telling jokes along the way. Take alcohol: 3.3 backlash + 7 impact x 4.5 J factor divided by 7 = 6.49.

Ergo, alcohol is bad for the Jews.

He then told a pretty good joke on the subject: A rabbi and a priest get into a nasty head-on collision, yet both emerge utterly unscathed. To celebrate the miracle, the rabbi hands the priest an open bottle of Manischewitz wine, from which he imbibes liberally. The priest hands the bottle back to the rabbi. “Won’t you have some?” he asked. “No,” said the rabbi, “I’ll wait for the police to arrive.”

Geller then passed around post-it notes and a fishbowl, asking his audience to submit their own GFTJ suggestions, for which he would then happily crunch the numbers.

(You do get that Geller totally makes this stuff up, right?).

One person asked whether cement is good for the Jews. It is. The Toyota Prius: not good for the Jews. Why? “When in doubt,” he said, “it’s not good for the Jews.”

He did wing an impressive bogus theory, however, regarding hybrid cars. “The faster we use up the oil,” he noted, “the sooner the Arabs will run out of it. So get guzzling oil!”

Monica Lewinsky: good for the Jews. Scientology: bad for the Jews (“Any religion that can’t laugh at itself isn’t much of a religion,” he said). Borat: good for the Jews (“He’s a Jewish character exposing anti-Semitism”). Easter: bad for the Jews.

In fact, said Geller, “Most things are not good for the Jews, but neither is asking the question.”

The conversation between Geller and the audience took on a more serious tone when the subject shifted to politics. He said that anti-Semitism persists in England mostly as anti-Israel screed. He also thought British society has not fully come to grips with its growing Muslim population, how to assimilate those people and how to deal with radical elements within them.

He also said that Renee Zellweger’s British accent in the two “Bridget Jones” movies was spot on.

This was Geller’s first U.S. appearance on a cross-country book tour, and judging by his easy give-and-take with the audience, his American tour will probably go a lot more smoothly than Borat’s.

He concluded by mentioning that the Guardian, one of England’s most strident newspapers and one considered anti-Israel, surprised him by recently serializing his book.

“I thought that was shocking,” he chuckled. “It was probably anti-Semitic.”

“Yes, But Is It Good For The Jews?” by Jonny Geller (288 pages, Bloomsbury, $15.95).