I am going by nibs.com's tipping measurements---I don't have the means to measure them myself---but I do know I like Pilot and Sailor's F nibs, and that's roughly the mark at which I'm shooting.
I was planning on acquiring the Carene in an XF in any case, but do you think I would have to have it reground to match that criteria? I don't have the option of trying it out first, mainly because international shipping back and forth would get costly. Also, I've noticed that the horizontal strokes on a Waterman XF (though this was on a Hemisphere) are slightly wider than the vertical ones: is that normal?
I don't know specifically how they are measuring the tip. Because what I do is quite similar. Except that I use a dial caliper rather than a micrometer.
What is not able to be measured is the effect of the tip profile/shape. With a round/ball tipping and a dry pen, the ink line would be the same size or narrower than the tipping, because of the small contact area. This is vs. a flattened tip which has a larger contact area.
I do not have a Waterman XF, but as I mentioned I think it will be close to a Pilot F.
The line variation you see is likely from manufacturing variation of the tipping.
If you buy the pen from nibs.com, you can specify that it be ground to match a Japanese F. Then specify which company, as the chart shows Pilot and Sailor are similar, but Platinum is different.