Sorry for any confusion. I'll try to clarify what I'm saying...
The way I'm using "break angle" is pretty standard and has been for years in discussing these matters, in my personal experience. So yeah, zztush is talking about what is happening on the opposite side of the nut that I'm talking about. At least that's what I think is happening. Indeed there isn't an angle on that side of things.
These are the terms as I've normally seen and heard them used over the years:
break point: The point where any given string touches the string nut or saddle.
break angle: The angle the string takes after it crosses
over the break point.
In the above
"over" would be towards the headstock if you're talking about the string nut. It would be towards the bottom of the instrument when speaking in terms of the saddle.
If the angle is too large, isn't all the pressure on the one side of the slot? The correct side of course and quite likely the string can take it. String being plastic the wear on that point would not be too excessive on the nut?
Yes. You got it. It is possible to wear the nut out faster if the angle is too steep, which leaves only a tiny point of contact between nut slot and string. Plain uke strings won't wear on a nut quite like steel strings for a guitar, even if the angle is a bit large.
If you just get as close to you can matching the way the strings break over the nut, it's usually perfectly fine.
This is an old website, but Frank Ford is one of the most respected tech/repairmen in the business and has been for years:
http://frets.com/FretsPages/Musician/GenSetup/Nuts/nuts1.html
That is page one of three in that section (you'll see at the bottom of the page where you can click "more" to go to the next page). He also uses simple terms and is easy to understand. I learned a lot from his frets.com site and a little from him personally in e-mail exchanges quite some years ago. I'm not sure how accessible he is now, as he has grown in popularity seems like. Even though the website looks outdated, it's still a gold mine for good, solid information.
One note on that section: There is a diagram with A, B, C, D nut slots on page three. He notes "A" as being ideal. I think that's most true for steel string instruments with strings that have some decent tension. Uke strings are lower tension and can sometimes pop out of their slot while playing if it's not a bit deeper than "A" in that diagram.
Lastly, there are different ways of doing things amongst techs...so you have to find what works for you within certain boundaries.