Calculate the percentage of legislators who are not Senator and were born before 1975.
not Senator refers to class is NULL; born before 1975 refers to birthday_bio < = 1975; calculation = MULTIPLY(DIVIDE(COUNT(class IS NULL THEN bioguide_id), COUNT(bioguide_id)), 1.0)
SELECT CAST(COUNT(CASE WHEN T2.class IS NULL THEN T1.bioguide_id ELSE NULL END) AS REAL) * 100 / COUNT(T1.bioguide_id) FROM current AS T1 INNER JOIN `current-terms` AS T2 ON T1.bioguide_id = T2.bioguide WHERE CAST(T1.birthday_bio AS DATE) <= 1975