Since you are only looking for an estimate of capacity remaining this should work:
DECLARE @DailyCapacity INT = 7950, -- Daily capacity
        @Minutes DECIMAL(18,2) = 660, -- Total minutes available per day
        @StartTime TIME = '08:00:00', -- Daily start time from capacity calculation
        @TestTime DATETIME = GETDATE();

SELECT @DailyCapacity, @Minutes, @StartTime, ROUND(@DailyCapacity * ((@Minutes - DATEDIFF(MINUTE,@StartTime,CAST(@TestTime AS TIME))) / @Minutes),0);