You should view MIL-HDBK-217 and similar methodologies as a “starting point” estimate. If your parts are “industrial grade”, then the use of a better quality factor than commercial may be warranted. If your experience indicates better reliability experience for these parts, then you should adjust accordingly – just document your assumptions and rationale.
Based on subsequent analysis of field data, I have often used a quality factor of 3 (one level better than commercial) for the “industrial” capacitors you are referring to. The failure rates in MIL-HDBK-217 have not been updated in a long time and for our experience, we have found that capacitors performed much better than MIL-HDBK-217 would indicate (in a ground benign environment).
