Best practitioners of the (modified) Vatican Roulette find it effective!

Christopher Mims of the Scientific American presents the key results in the second and third paragraphs:

A new German study, however, has found that, when practiced correctly, a method of periodic abstinence known as the sympto-thermal method (STM) leads to an unintended pregnancy rate of only 0.6 percent annually. This rate is comparable with that of unintended pregnancies in women who use birth control pills, the most popular method of contraception in the U.S.

For the sympto-thermal method to work, women must keep track of three things: their core body temperature, the fertile days of their cycle as measured by a calendar, and their cervical secretions. Using this information, women are able to abstain from sex during their fertile period, which is the two weeks that surround the day on which they ovulate. According to lead study author Petra Frank-Herrmann, a fertility researcher at the University of Heidelberg in Germany, STM is more effective than the other so-called periodic abstinence methods because it uses more than one type of information to predict the dates of a woman's fertile period.

The rest of the story is used to point out how STM cannot work for everyone, and in particular, young couples: As a women's health expert says in the story, "It's difficult to abstain from sex for two out of four weeks. That means half the month you can't have sex. That's very difficult for young couples"

The best summary of the story appears right in the first paragraph:

For years the birth control methods collectively known as periodic abstinence have been jokingly referred to as "Vatican roulette," a nod to the fact that these techniques are both Vatican approved and quite likely to end in pregnancy.