Next Numbers Reference

Can anyone provide me with a reference site or document containing a formulae or algorithm for the Address number (an8) Next numbers table. Third Party software for validation requires this to complete check digit validation logic. All help gratefully received.

Best Wishes

MrG

Done a little more digging to find that the next numbers routine is supported by the modulus 11 ideal. however, I don't know what factors apply within each bracket. Any body, any references? - or just the answer, even better

Attached is an excel spreadsheet that I used to calculate the check digit

Thanks, this has worked for address numbers starting 5... but anything previous to this it appears won't work as the check digit facility I believe was not active at this point. Therefore I am unlikely to find a generic algorithm for these check digits!