Calculate the percentage of the nominees who were born in USA.
nominees refers to result = 'Nominee'; born in USA refers to birth_country = 'USA'; percentage = divide(sum(result = 'Nominee' and birth_country = 'USA'), count(Person.name)) * 100%
SELECT CAST(SUM(CASE WHEN T1.birth_country = 'USA' THEN 1 ELSE 0 END) AS REAL) * 100 / COUNT(*) FROM Person AS T1 INNER JOIN Award AS T2 ON T1.name = T2.person WHERE T2.result = 'Nominee';