Question: What percentage do items under the category of 'Furniture' make up the total number of items ordered that are shipped as standard in the West region?
SQL: SELECT CAST(SUM(CASE  WHEN T2.Category = 'Furniture' THEN 1 ELSE 0 END) AS REAL) * 100 / SUM(T1.Quantity) FROM west_superstore AS T1 INNER JOIN product AS T2 ON T1.`Product ID` = T2.`Product ID` WHERE T2.Region = 'West' AND T1.`Ship Mode` = 'Standard Class'