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