Watch O-Ring Selecting Round Watch O-Ring Gasket Size, uk.

Choosing the correct flat size gasket.
After removing the back case cover take the inside measurements across the back of the case where the lip or the thread is situated, then take the outside measurements across the whole of the back case, now reduce the outside determine by 0.2mm,. this should give you the correct size gasket. Recommendation use a pair of digital callipers to take the measurements