CREATE TABLE T (a int, b int, c int); 
GO
INSERT INTO T VALUES (1, 1, -3), (2, 2, 4), (3, 1, NULL), (4, 3, 1), (5, 2, NULL), (6, 1, 5); 

SELECT b, c, 
    LEAD(2*c, b*(SELECT MIN(b) FROM T), -c/2.0) OVER (ORDER BY a) AS i
FROM T;