As of v0.23.3, WikimediaUI theme's rules set margin-top of fieldLayouts to 15px on consecutive and inline ones and 21px on others.
It is possible to
- not negatively impacting visual proximity principle,
- stay harmonious with the label header to widget distance (currently 4px),
- not negatively impacting touch area minimum sizes (distances) and
- use a multiplier of 8px, chosen as our base value for vertical rhythm