Percent decrease in value each month with SQL
You can use a function/formula:

CREATE FUNCTION Sequence(@Date date)
RETURNS decimal(5,2) 
AS
BEGIN

DECLARE 
    @InitYear int = 2019,
    @InitValue decimal (5,2) = 93.60,
    @Decrement decimal (5,2) = 0.20,
    @MinValue decimal (5,2) = 90.60,
    @Result decimal(5,2) = 0;

    SET @Result = (@IniTValue 
                  - ((DATEPART(year, @Date) - @InitYear) * 12 * @Decrement))
                  - ((DATEPART(month, @Date)-1) * @Decrement);

    IF (@Result &lt; @MinValue)
        SET @Result = @MinValue;

    RETURN @Result;
END




SELECT dbo.Sequence ('20190101') AS '01/01/2019',
       dbo.Sequence ('20190201') AS '01/02/2019',
       dbo.Sequence ('20190301') AS '01/03/2019',
       dbo.Sequence ('20190401') AS '01/04/2019',
       dbo.Sequence ('20190501') AS '01/05/2019',
       dbo.Sequence ('20190601') AS '01/06/2019';

SELECT dbo.Sequence ('20200101') AS '01/01/2020',
       dbo.Sequence ('20200201') AS '01/02/2020',
       dbo.Sequence ('20200301') AS '01/03/2020',
       dbo.Sequence ('20200401') AS '01/04/2020',
       dbo.Sequence ('20200501') AS '01/05/2020',
       dbo.Sequence ('20200601') AS '01/06/2020';




01/01/2019 | 01/02/2019 | 01/03/2019 | 01/04/2019 | 01/05/2019 | 01/06/2019
:--------- | :--------- | :--------- | :--------- | :--------- | :---------
93.60      | 93.40      | 93.20      | 93.00      | 92.80      | 92.60     

01/01/2020 | 01/02/2020 | 01/03/2020 | 01/04/2020 | 01/05/2020 | 01/06/2020
:--------- | :--------- | :--------- | :--------- | :--------- | :---------
91.20      | 91.00      | 90.80      | 90.60      | 90.60      | 90.60     



db&lt;>fiddle here