If you are really performing work to AWS D1.1 then there is no need to worry about the customer rejecting work. First there is no limit on over sized welds. There are two issues that can restrict weld size. This would be heat input and accessibility (or allowing all parts to fit together). Undersized welds need to be within tolerances, I think this is - 1/16" for 10%. I have not come across your + or - 15% tolerance that you mentioned in the AWS inspection book? Do you have a reference? I would never accept that a customer would reject parts that were made to the applicable code. That is bad business. Maybe it's just me but you seem just as fuzzy in your understanding as well.