How to Calculate Bank Routing Check Digit

CREATE FUNCTION create_check_digit (@routing_number char(8) ) returns int
as
begin

declare @digit int
,@sum_digit int

declare @check_digit int
set @check_digit=0

--1
set @digit =convert(int, substring(@routing_number, 1, 1))
set @sum_digit = @digit*3
--2
set @digit =convert(int, substring(@routing_number, 2, 1))
set @sum_digit = @sum_digit + @digit*7
--3
set @digit =convert(int, substring(@routing_number, 3, 1))
set @sum_digit = @sum_digit + @digit*1
--4
set @digit =convert(int, substring(@routing_number, 4, 1))
set @sum_digit = @sum_digit + @digit*3
--5
set @digit =convert(int, substring(@routing_number, 5, 1))
set @sum_digit = @sum_digit + @digit*7
--6
set @digit =convert(int, substring(@routing_number, 6, 1))
set @sum_digit = @sum_digit + @digit*1
--7
set @digit =convert(int, substring(@routing_number, 7, 1))
set @sum_digit = @sum_digit + @digit*3
--8
set @digit =convert(int, substring(@routing_number, 8, 1))
set @sum_digit = @sum_digit + @digit*7

if (@sum_digit % 10) > 0 
set @check_digit = 10 - (@sum_digit % 10)

return @check_digit
end