[ft] How to compute ascent and descent for Latin characters?

[Anson]

Has anyone ever tried to compute the ascent and descent for Latin
characters?  I.e. 'A' to 'Z' and 'a' to 'z'?

The reason I am asking this is that all metrics I have come across are
defined on _all_ glyphs in a given font file.  Apparently their values
are greater than those of Latin characters because western fonts
usually include characters like Å and ş.  The only exception I could
find are os2.usWinAscent and os2.usWinDescent, which are still defined
to include characters in the Windows ANSI character set.

Does FreeType provide a standard 'cunning' way to compute metrics for
Latin-only characters?  If not what would be the best way to do it?

Another (unrelated) question: what is "AFM file" as mentioned in
os2.sTypoAscender and os2.sTypoDescender documentation?

Any help would be much appreciated.

Anson