A 52-year-old hypertensive woman presents with sudden-onset thunderclap headache described as "the worst headache of her life," accompanied by brief loss of consciousness, vomiting, and photophobia. On examination, she has neck stiffness and Kernig sign. Non-contrast CT shows hyperdense blood in the basal cisterns, sylvian fissures, and interhemispheric fissure. CT angiography of the circle of Willis is performed urgently and reveals a saccular aneurysm at the location marked **A**. According to epidemiological data on ruptured saccular berry aneurysms, what is the approximate percentage of cases that occur at the site marked **A**?