From Tailwind I've learnt that 4px is the ideal base unit. You'll almost never want to go below 2 (0.5) pixels for anything in spacing and thin borders can just be 0.25 of base. The standard font size of 16 pixels is 4 units.
4 just seems to be the perfect CSS number.
From Every Layout I've learned the concept of Cluster and Stack. Although I don't use these literally from the book conceptually I now see all horizontal (row) Flexbox layouts as Clusters and vertical (column) as Stacks and use a utility class to define these as base elements in my layouts.