Rounding script works to the nearest 15 minutes based on duration, how to shift this threshold by negative 30 seconds for decimal hours

I have a script which rounds a time length to the nearest 15 minutes. So for example 00:23:00 minutes to 00:37:59 duration will return 0.5 (decimal 30 minutes) etc continuous around the clock. I am trying to now get the script to work in half minutes so I need to shift this by 30 second offset for the rounding, so 00:22:29 seconds will go to 0.25 and 00:22:30 will round to 0.5. Here is the original script. So this needs to return 0.5 not 0.25. If you change the @finish to 08:22:29 it needs to then return 0.25. Thank you

declare @start datetime = '2020-03-01 08:00:00:000'
declare @finish datetime = '2020-03-01 08:22:30:000'

select
(CAST(DATEPART(hh, (DATEADD( minute, ( DATEDIFF( minute, 0, DATEADD( second, ( 15 * 60 ) / 2, (convert(varchar(5),(@finish - @start),108)) ) ) / 15 ) * 15, 0 ))) AS float) +
    CAST(DATEPART(mi, (DATEADD( minute, ( DATEDIFF( minute, 0, DATEADD( second, ( 15 * 60 ) / 2, (convert(varchar(5),(@finish - @start),108)) ) ) / 15 ) * 15, 0 ))) AS float) / 60 +
        CAST(DATEPART(ss, (DATEADD( minute, ( DATEDIFF( minute, 0, DATEADD( second, ( 15 * 60 ) / 2, (convert(varchar(5),(@finish - @start),108)) ) ) / 15 ) * 15, 0 ))) AS float) / 3600)

Answer

Here’s another possible solution, using CASE to classify the modulo seconds over an hour:

SELECT DATEDIFF(second,@start,@finish) / 3600 +
CASE WHEN DATEDIFF(second,@start,@finish) % 3600 <  450 THEN 0.0
     WHEN DATEDIFF(second,@start,@finish) % 3600 < 1350 THEN 0.25
     WHEN DATEDIFF(second,@start,@finish) % 3600 < 2250 THEN 0.5
     WHEN DATEDIFF(second,@start,@finish) % 3600 < 3150 THEN 0.75
     ELSE 1
END