The KISS principal should be followed
The idea is not to get pilots to think too much, else they don't read the manuals. The safe side to be on is the 10 degree safety bar in visible moisture. As was stated earlier when engines suck they depress the temp in the inlet.
Lots of problems with ice forming in engines and not on the wiper blades etc.
When I last checked, all the manuals for consistency in the big iron recognized this. I'll keep my eyes tuned for changes in this forum and check this out if the manuals are not consistent.