I don't mean the stock one from Dayton when I say "generic", all the calibrations I've seen from Dayton are flat out awful. By "generic" I mean, if they were fairly consistent mic-to-mic and you did a calibration from one basically "in the middle of the pack" - well that could be offered as a "generic" calibration file, and potentially, be 10x better than the ones generated specifically for each mic from the Dayton website. Generic calibration files for Berhinger measurement mics have been passed around in the past. I just don't know for sure because I have no idea how similar UMM-6 mics generally are unit-to-unit.I only have a single Mic. No way to compare or know the lot to lot variation.
I would try to speak with CS to get an idea how much variation has been seen.
I would guess that for 99% of the folks out there the stock calibration is totally fine. In my case I was building a sub so wanted the subsonic accuracy.
Good luck and Happy New Year!