*This figure (99.6% effective) is based off a 2007 clinical trial out of Germany. Study authors followed 900 women across 17,638 menstrual cycles. Study participants were working closely in person with instructors of the Sensiplan method. With perfect use of this particular Symptothermal Method (Sensiplan), participants prevented pregnancy with 99.6% effectiveness. I do not teach the Sensiplan method, but teach a very similar method which is a hybrid of some aspects of Sensiplan and some aspects from my training with the Natural Family Planning Teachers Association of the United Kingdom. You can read more about the method I teach when you click through to your preferred learning format.
Typical use effectiveness rates for the Symptothermal Method are less clear as we lack high enough numbers of users in the real world. You can read more about this here.