Calculate the percentage of legislators who are Senator and were born in 1964.
are senator refers to class IS NOT NULL; born in 1964 refers to birthday_bio = 1964; calculation = MULTIPLY(DIVIDE(COUNT(class IS NOT NULL THEN bioguide_id), COUNT(bioguide_id)), 1.0)
SELECT CAST(SUM(CASE WHEN T2.class IS NOT NULL THEN 1 ELSE 0 END) AS REAL) * 100 / COUNT(*) FROM current AS T1 INNER JOIN `current-terms` AS T2 ON T1.bioguide_id = T2.bioguide WHERE T1.birthday_bio LIKE '%1964%'