Calculate the percentage of total payment of MARGARET MOORE customers.
percentage = divide(sum(amount where first_name = 'MARGARET' and last_name = 'MOORE'), sum(amount)) * 100%
SELECT CAST(SUM(IIF(T2.first_name = 'MARGARET' AND T2.last_name = 'MOORE', T1.amount, 0)) AS REAL) * 100 / SUM(T1.amount) FROM payment AS T1 INNER JOIN customer AS T2 ON T1.customer_id = T2.customer_id