What percentage of people have worked on the True North episode as additional crew?
the True North episode refers to title = 'True North'; additional crew refers to role = 'Additional Crew'; percentage = divide(count(episode_id where role = 'Additional Crew'), count(episode_id)) * 100% where title = 'True North'
SELECT CAST(SUM(CASE WHEN T2.role = 'Additional Crew' THEN 1 ELSE 0 END) AS REAL ) * 100 / COUNT(T1.episode_id) FROM Episode AS T1 INNER JOIN Credit AS T2 ON T1.episode_id = T2.episode_id WHERE T1.title = 'True North'