Full disclosure: My automotive experience pre-dates IATF 16949, but I do consider myself knowledgeable in MSA. I recommend challenging your auditor on this. The usual terminology used for the 10-30% range is that the gage MAY be acceptable depending on the application, the cost of the measurement device, etc. There are a number of reasons that you can use to justify using a gage in this range:
- Is the feature/dimension a significant/critical/CTQ characteristic? If no, the gage may be used.
- Is the gage the state of the art? If yes, the gage may be used.
- Is the next better gage extremely expensive? If yes...
- ...
I would add these criteria (or those relevant to your business) to your procedure/documentation for MSA.