SQL Query to get Percentage of Elapsed Time calculating total time and time to next row, Sql Server

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IF OBJECT_ID('dbo.mytable', 'U') IS NOT NULL 
BEGIN
   DROP TABLE dbo.mytable
END

CREATE TABLE mytable
    ([TimeStamp] datetime, [State] varchar(9))

INSERT INTO mytable
    ([TimeStamp], [State])
VALUES
    ('2015-04-27 09:49:14', 'SETUP'),
    ('2015-04-27 10:10:07', 'EXECUTING'),
    ('2015-04-27 14:15:12', 'WAITING'),
    ('2015-04-27 14:15:48', 'EXECUTING'),
    ('2015-04-27 14:29:10', 'WAITING'),
    ('2015-04-27 14:29:38', 'EXECUTING'),
    ('2015-04-28 01:58:54', 'WAITING'),
    ('2015-04-28 02:03:25', 'CLOSED')

;WITH CTE_DIFF AS (
   SELECT [TimeStamp], [State], 
          DATEDIFF ( second , 
                    [TimeStamp] , 
                    LEAD([TimeStamp]) OVER (ORDER BY [TimeStamp])) AS time_diff 
   FROM mytable
), CTE_PERC AS (
   SELECT [TimeStamp], [State], time_diff ,
          SUM(time_diff) OVER (ORDER BY [TimeStamp]) * 1.0 / SUM(time_diff) OVER () * 100 AS perc
   FROM CTE_DIFF
)
SELECT [TimeStamp], [State], 
       COALESCE(LAG(perc) OVER (ORDER BY [TimeStamp]), 0) AS PercentageStart,
       perc AS PercentageEnd
FROM CTE_PERC

DROP TABLE dbo.mytable

View schema
Execution time: 0,03 sec, rows selected: 8, rows affected: 8, absolute service time: 0,19 sec

SQL Query to get Percentage of Elapsed Time calculating total time and...