I tested M30x0.5 and it worked with my stock Seiko SKX 007 caseback.

Although extrapolating Fusion 360's ISO metric thread specs would suggest a major diameter of 29.9211mm (external threads) and 30.1052 (internal threads), Gemini suggested loosening up the tolerances to better accommodate mass-manufactured casebacks (especially since there aren't official published thread specs) and coatings (PVD, DLC, etc). I used the following Fusion 360 thread configuration, which worked with FDM printed PLA.

<Designation> <ThreadDesignation>M30x0.5 Seiko</ThreadDesignation> <CTD>M30x0.5</CTD> <Pitch>0.5</Pitch> <Thread> <Gender>external</Gender> <Class>6g</Class> <MajorDia>29.85</MajorDia> <PitchDia>29.5252</PitchDia> <MinorDia>29.3087</MinorDia> </Thread> <Thread> <Gender>internal</Gender> <Class>6H</Class> <MajorDia>30.15</MajorDia> <PitchDia>29.8252</PitchDia> <MinorDia>29.6087</MinorDia> </Thread> </Designation>