Fortunately, probabilities for birth control methods aren't presented in this way. Generally, when an organization like Planned Parenthood says that the symptothermal method is 99.6% effective, what they really mean is:

"Of 100 couples who use the symptothermal method correctly for one year, 0.4 (fewer than one) will have a pregnancy." (Source: http://www.plannedparenthood.org/health-topics/birth-control...)

So after about 100 years of using this method, you've got a .4% chance of becoming pregnant. Seems like decent odds to me.