What Tyrone says about the 29 or 29.5 cut is what I have always read, but I suspect the real reason is just a bit deeper.
Consider your comments about the marks on your compound being more than 1/2 degree wide and your doubts about being able to set it that accurately. I suspect that this is true for most lathes to one degree or another. There is no Vernier scale on the compound and I would bet that +/- 1/2 degree is the best any lathe manufacturer would be likely to claim if they were to actually write a spec on it. Oh, and then there is the tolerance on the angle of the lathe bit used to cut it. Just how sure are you that it is exactly 60 degrees? Perhaps it is 59 or 61. How many shops have the ability to accurately measure an angle with such short sides?
So, with at least a half degree of doubt in the actual angle the compound is set to, if you set it to the 30 degree mark, you are just as likely to be at 30.5 as you are to be at 30 or 29.5. If you cut a 60 degree thread at 29.5, theoretically one flank of thread will show a set of steps from each successive pass and this is not desirable. So, by setting the compound to a slightly smaller angle, you are much more confident that both flanks of the thread you are cutting are smooth and not stepped.
I don't think it really makes any difference weather you are cutting at exactly the angle of the flank or at a slightly smaller angle which will shave a bit more off on each pass. But if you are at a slightly greater angle, you will leave a set of steps on one flank. So it is better to err in the direction of a smaller angle instead of a greater one.
Make it fit.
You can't win and there is a penalty for trying!