Since your question asked what is the smallest minimum annular ring, I know of
one very complex 52 layer PWB that is designed and built using a pad size that
is 6-mils larger than the drill size, giving nominal  annular ring of 3-mils.
6-mils over the drill size is a nice rule of thumb for the "smallest minimum"
annular ring.

Regards,
Lyle Wallig

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