Many people here give advice like that. The advice could well be OK in many cases. But in other cases it could be off by enough to make someone end up with an overloaded recording despite their best efforts to avoid it.
I also don't understand your post. However, my response is not to debate your post on technical merit. I think you're missing the point of how people, for better or worse, are applying the unity gain term.
I believe his point is that what many folks call "unity gain" is not, in fact, unity gain. Nor is unity gain (as correctly defined) a particularly important thing for our purposes. I thought most engineers were sticklers for correct terminology.
I think that your explanation, which I don't get, overcomplicates a simple concept which IMHO, for 99% of users, doesn't need any technical elaboration. If I use a Sony M10 and it overloads at a loud rock show with a setting of 4, then the next time I set it at 3. If my explanation for the distortion was that my setting was above 'unity gain', whether technically accurate or not, why does it matter?
Why not just call it what it is - the minimum safe setting for the input level? Rather than unity gain, that has an actual meaning, but that actual meaning is different.
Also, how do you know setting it at 3 next time will help? Perhaps what you should be learning about your gear is that if the levels are too hot at 4, you need an external attenuator, turning it down to 3 won't help. (This is arbitrary, I don't know what the safe level is on an M10. But I do know that with a Tascam DR2D's mic in, if your levels are too hot at 67, don't bother turning it down to 60 - you need an external attenuator.)
The only reliable way to find the lowest safe setting is first to find the actual input overload point with a signal generator--
Really? Who has access to a signal generator? Not to be cynical, but why would I bother and why do I care?
Just about anybody reading this site has access to a signal generator. Any decent audio editing software can generate all the test tones you could every need, and most of us have our computers set up for reasonable playback. In terms of actual voltages, who cares indeed. But it can be very useful to fire up a test tone (so you can very easily recognize distortion in the recorded wav file), send it into your recorder at various levels, and see at what input levels you can get an unclipped waveform back and what levels you can't, even if the recording meters aren't hitting "over" as you record. (Do be aware though that some recorders can take hotter inputs than a lot of consumer gear can put out without clipping, so you do have to be sure you're overloading your recorder and not simply overdriving your source).
Perhaps people are using some poetic license then if they were to call '4' the unity gain point because they haven't done the bench testing you suggest, but really why is that such a big deal in the context it's been used?
I hope I don't come off as a jerk with my responses above, but I'm just not getting your points about why the concept of unity gain, as it's been used here in the past, is a myth and why it's not valid.
I think the problem is that people are conflating minimum safe input levels with "unity gain", where the output voltage is equal to the input voltage. But what really matters for recording is making sure that you don't overload any intermediate stages hidden deep inside the recorder. I agree that in practical terms it is not the actual numeric value of the voltage that matters - it's the record setting that is safe. But that is not unity gain!
There also seems to be a myth that at the magical "unity gain" setting, you are bypassing all of the internal gain stages of the recorder and getting something purer. With most consumer gear, that is not the case - there are usually gain stages that you just can't bypass, so there's nothing magical about unity gain on a typical digital pocket recorder. The concern should be minimum safe input levels, and knowing that you will need an external attenuator if the incoming signal is too hot at those minimum levels.