Find top by values by percents using range in SQL Server
Another way of doing it would be

WITH CTE
     AS (SELECT ID,
                PKI,
                ROW_NUMBER() OVER ( ORDER BY PKI) AS RN,
                COUNT(*) OVER()                   AS Cnt
         FROM   myTable)
SELECT ID,
       PKI
FROM   CTE
WHERE  RN BETWEEN 0.5 * Cnt AND 0.75 * Cnt