Sample, Sql Server

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CREATE TABLE Events
    ([Flow] float, [Head] varchar(4), [Series_Name] varchar(1))
;
    
INSERT INTO Events
    ([Flow], [Head], [Series_Name])
VALUES
    (0, NULL, 'A'),
    (0, NULL, 'B'),
    (10, NULL, 'A'),
    (10, NULL, 'B'),
    (20, NULL, 'A'),
    (22.5, '88', 'B'),
    (20, NULL, 'B'),
    (30, NULL, 'A'),
    (30, NULL, 'B'),
    (39.42, '60.1', 'A'),
    (40, NULL, 'A'),
    (40, NULL, 'B'),
    (50, NULL, 'A'),
    (50, NULL, 'B'),
    (60, NULL, 'A'),
    (60, NULL, 'B'),
    (70, NULL, 'A'),
    (70, NULL, 'B'),
    (80, NULL, 'A'),
    (80, NULL, 'B'),
    (90, NULL, 'A'),
    (90, NULL, 'B'),
    (100, NULL, 'A'),
    (100, NULL, 'B'),
    (110, NULL, 'A'),
    (110, NULL, 'B'),
    (120, NULL, 'A'),
    (120, NULL, 'B')
;

SELECT *
from Events
WHERE Flow <= (SELECT max(Flow) + 3*10 FROM Events WHERE [HEAD] IS NOT NULL);

WITH addRow_id as (
     SELECT *, ROW_NUMBER() OVER ( ORDER BY [Flow], [Series_Name]) as rn
     FROM Events
),
lastID as (
     SELECT MAX(rn) as last_row
     FROM addRow_id
     WHERE [HEAD] IS NOT NULL
)
SELECT *
FROM addRow_id A
CROSS JOIN lastID L
WHERE A.rn <= L.last_row + 6;

View schema
Execution time: 0,03 sec, rows selected: 32, rows affected: 28, absolute service time: 0,2 sec