In the real world keep it simple…
For simple pilots and de-icers. Yes they are in the real world! Frost on the uppers is a no go, no matter what the thickness or area it is in. I am sure many of you like myself have seen Ice/frost spread like cancer across the wing after inspecting aircraft on arrival, and just before departure. The fresh/warmer fuel temp did not melt it that time!
A lot depends on the temp (of course), the wind direction, parking location, and the way the apu hot air is blowing - is another e.g.
Many a time the crew do not look at the wings from the steps or windows with this in mind. What about that one time the aircraft is delayed that little bit longer, and they go with the attitude of - nah we will be ok? and it aint!
Boeing says they allow it in some in areas…blah…blah!
off ! You get on the wing and measure it with a ruler matey! You can do a tactile test for most of the wing (without risking some damage with a basket, etc). Always cop a feel if you have any doubts. Shinny metal can look ok. But it could be covered in a thin layer of ice.
If in doubt de-ice it!
Procedures - Airlines all have their own. You would need a book the size of War and Peace on you for reference going airside.
Again keep it simple and the same rule for all types of aircraft.