Convert sub-query to windowed aggregate function ("WinMagic")

SELECT SUM(l_extendedprice) / 7.0 AS avg_yearly
FROM   tpcd.lineitem, tpcd.part
WHERE p_partkey = l_partkey
AND p_brand = 'Brand#23'
AND p_container = 'MED BOX' 
AND l_quantity < (
    SELECT 0.2 * avg(l_quantity)
    FROM tpcd.lineitem
    WHERE l_partkey = p_partkey)

WITH WinMagic AS (
    SELECT l_extendedprice, l_quantity,
        avg(l_quantity) OVER (PARTITION BY p_partkey) AS avg_l_quantity
    FROM  tpcd.lineitem, tpcd.part
    WHERE p_partkey = l_partkey
    AND p_brand = 'Brand#23'
    AND p_container =  'MED BOX')
 SELECT SUM(l_extendedprice) / 7.0 as avg_yearly
 FROM WinMagic
 WHERE l_quantity < 0.2 * avg_l_quantity;