Same with post #3, differentiating well between
- freezing conditions (OAT < 10 deg C, active precipitation, spray, visible moisture with VIS below one mile (=FG))
- freezing fog (OAT sub zero)
would probably help during the original decision making. Much easier to see from a comfy chair, sure. Wing skin temperature could overrule it all, as well covered in #3.
I would have probably asked for Type I 25:75, partly to remove any leftover non-conforming Type II, partly because I believe the mantra doubt = de-ice. Smart F/O might have tipped the scales either way, both sides really, likewise if overly smart to show where the line is drawn. Passing on freely what once was given.
If it had been my personal airplane with me on it single-pilot, then no additional treatment as neither the conditions for icing nor physical risk of ice formation were there. Again, based on what's reported, wearing my keyboard battledress, and only if the fuel temperature / wingskin temp. assessment would allow.
To answer the question placed:
The industry standard for winter ops in this regard is actually produced by IATA: Deicing Anti-icing Manual. Comes updated every winter with a new annual edition. Cannot find the link.
ICAO is in the making:
https://www.skybrary.aero/bookshelf/books/4400.pdf
FAA:
https://www.faa.gov/other_visit/avia...overTables.pdf