SQL Server - Encontrar dia entre fechas

LEADTIME   |   DESPACHO ESTIMADO   |   ENTREGA   |   DIAS RETRASO

1                 06/10/2015          09/10/2015    (Para calcular)

5                 08/10/2015          10/10/2015    (Para calcular)

4                 09/10/2015          12/10/2015    (Para calcular)

create function fn_FindSunday (@date1 datetime, @date2 datetime)

returns bit

as

 begin

 declare @bit int

 set @bit = 0

  while @date2 < @date1

   begin

    if DATEPART(dw,@date1) = 7

     begin

     set @bit = 1

     break

     end

    select @date1 = DATEADD(day, 1, @date1)

    continue

   end

   return(@bit)

  end

SELECT diasretraso = datediff(dd,[despacho estimado],entrega) - (leadtime + dbo.fn_FindSunday([despacho estimado], entrega))

from #test