How to Validate Bank Routing Using Check Digit

CREATE FUNCTION check_digit (@routing_number char(9) ) returns int
as
begin

declare @digit int
,@sum_digit int

declare @check_digit int
set @check_digit=0

--1
if isnumeric(substring(@routing_number, 1, 1))=0 return -991
set @digit =convert(int, substring(@routing_number, 1, 1))
set @sum_digit = @digit*3
--2
if isnumeric(substring(@routing_number, 2, 1))=0 return -992
set @digit =convert(int, substring(@routing_number, 2, 1))
set @sum_digit = @sum_digit + @digit*7
--3
if isnumeric(substring(@routing_number, 3, 1))=0 return -993
set @digit =convert(int, substring(@routing_number, 3, 1))
set @sum_digit = @sum_digit + @digit*1
--4
if isnumeric(substring(@routing_number, 4, 1))=0 return -994
set @digit =convert(int, substring(@routing_number, 4, 1))
set @sum_digit = @sum_digit + @digit*3
--5
if isnumeric(substring(@routing_number, 5, 1))=0 return -995
set @digit =convert(int, substring(@routing_number, 5, 1))
set @sum_digit = @sum_digit + @digit*7
--6
if isnumeric(substring(@routing_number, 6, 1))=0 return -996
set @digit =convert(int, substring(@routing_number, 6, 1))
set @sum_digit = @sum_digit + @digit*1
--7
if isnumeric(substring(@routing_number, 7, 1))=0 return -997
set @digit =convert(int, substring(@routing_number, 7, 1))
set @sum_digit = @sum_digit + @digit*3
--8
if isnumeric(substring(@routing_number, 8, 1))=0 return -998
set @digit =convert(int, substring(@routing_number, 8, 1))
set @sum_digit = @sum_digit + @digit*7

-- last one
if isnumeric(substring(@routing_number, 9, 1))=0 return -999
set @digit =convert(int, substring(@routing_number, 9, 1))
--select  substring(@routing_number, 9, 1), len(@routing_number)

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

if @digit<> @check_digit
begin
--print 'Invalid routing =' + @routing_number
--select @digit, @check_digit
select @check_digit=-99
end

return @check_digit
end