Write SQL query to solve given problem: What is the percentage of Professor Ogdon Zywicki's research assistants are taught postgraduate students?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT CAST(SUM(CASE WHEN T3.type = 'TPG' THEN 1 ELSE 0 END) AS REAL) * 100 / COUNT(T1.student_id) FROM RA AS T1 INNER JOIN prof AS T2 ON T1.prof_id = T2.prof_id INNER JOIN student AS T3 ON T1.student_id = T3.student_id WHERE T2.first_name = 'Ogdon' AND T2.last_name = 'Zywicki'