Thanks to all who chimed in.
In the meantime, it has struck me, that my "method", to the degree it could be made precise, would have the same result as using the grey card.
When I started shooting digital, I took a purist approach and set every shot to daylight. This gave trustworthy, if uncontrolled results over a wide range. But some images shot under overcast sky late in the afternoon or early summer evening were absolutely too blue for my memory.
After that, I have relied entirely on the AWB of the camera and then chosen As Shot in the raw converter. The point of departure for my current attempt was that in some cases I discovered that the simple Daylight setting in the raw converter was more conform to my (fresh) memory than the AWB/As Shot combo. I also started wondering how the tint numbers actually were made up, hence my initial question.
Czornyjs answer seemed to make things a little simpler by omitting the tint. Erics post on the contrary points out that matters are even more complex. The whole spectrum of light would need to be considered. "So, a global temp/tint white balance is really a ballpark estimate to get you started." The problem is: started with what?
The concept of mixing in saturation is all new to me, I have no idea how it fits in theoretically. What happens to color saturation, compared to "natural"?
I have no problem making an image look "natural" just by changing the WB - my problem is that it looks "natural" over a wide range, and I would like a little more precision.
- I just tried one image which was shot late afternoon and which I remember I found too blue/green when my memory was fresh. Result:
1-Strange enough, the image does not look unnatural to me today at 5200 K=as shot.
2-It becomes even more "natural" if I increase exposure by 1/2 f-stop - however, the late afternoon mood is lost to some degree.
2-My memory, now 5 years old, accepts WB up to about 7000K, if I insist, that the thing did in fact look green. 7000K is probably pretty close to the temp at shooting time.
3- increasing saturation makes the image more blue/green.
No I will not carry additional instruments. Everything that weighs has to be omitted. One problem such instruments would meet is that my shooting "biotope" these days is woodland, and there is so to speak never an unobstructed view to the light source.
Even if there was, and even if the camera had a built-in spectrometer, there would be one problem left, same as with the grey card: The "AWB" of our brain does not work 100%, and hence, no objective measurement of incident light would have to be followed 100%. But to which %?
For now, I think I will try the following: Shoot with AWB. Then set the raw converter to daylight, say 5200 K. Neglect tint. If the resulting image is in all too strong contrast to my fresh memory, I'll try to adjust temp, staying as close to 5200 K as my memory will permit.