Demo, Sql Server - rextester

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CREATE TABLE t(
   Customer INTEGER  NOT NULL PRIMARY KEY 
  ,[User]     VARCHAR(5) NOT NULL
  ,Revenue  INTEGER  NOT NULL
);
INSERT INTO t(Customer,[User],Revenue) VALUES
 (001,'James',500)
,(002,'James',750)
,(003,'James',450)
,(004,'Sarah',100)
,(005,'Sarah',500)
,(006,'Sarah',150)
,(007,'Sarah',600)
,(008,'James',150)
,(009,'James',100);

WITH cte AS
(
    SELECT c.Customer, c.[User], c.[Revenue]
           ,percentile         = 1.0 * Revenue / NULLIF(c3.s,0)
           ,running_percentile = 1.0 * c2.s    / NULLIF(c3.s,0)
    FROM t c
    CROSS APPLY
         (SELECT SUM(Revenue) AS s
          FROM t c2
          WHERE c.[User] = c2.[User]
            AND c2.Revenue >= c.Revenue) c2
    CROSS APPLY
         (SELECT SUM(Revenue) AS s
          FROM t c2
          WHERE c.[User] = c2.[User]) AS c3
) 
SELECT a.*
FROM cte a
CROSS APPLY (SELECT MIN(running_percentile) AS rp
             FROM cte c2
             WHERE running_percentile >= 0.8
               AND c2.[User] = a.[User]) AS s
WHERE a.running_percentile <= s.rp
ORDER BY [User], Revenue DESC

View schema
Execution time: 0,02 sec, rows selected: 5, rows affected: 9, absolute service time: 0,19 sec