I appreciate the effort to accommodate kachibi's request, but I don't think it makes sense to quantify these words so precisely. They are all subjective. For example, in the Kenkyusha dictionary, 大抵 is defined as: (大部分の) a good part; nearly all; almost all; (ほとんどの) nearly [almost, practically] all [every].
As you can see, there is great overlap in the meanings. In English, "most" can be anything over 50% in some cases, but in other cases it refers to a higher proportion. The same applies to these adverbs in Japanese (also including 大半).
To me, it makes more sense to say that 大体 is probably the weakest of the three, while the other two are similar in strength, and one may be preferred over the other depending on the exact context.
In a textbook, I once leaned the range of percentage regarding to English adverbs such as: perhaps, probably, maybe, mostly, major*, minor*, etc. (I don't remember the precise number now.) *adjecitve
The author tried to ask 10 native speakers about such frequencies.
And he learned it was difficult to indicate the range because each native speaker has their different opinion. Yet, he showed the "mean" range for Japanese learners in order to help their understanding.
I asked my English tutor about the textbook, and he denied its range, saying he didn't agree with it.
Then, I learned that the Showing-percentage-explanation has limitation to their accuracy. It is merely a figurative speech to get along with understanding foreign languages.
What I did in #2 here was the same attempt like the author of that textbook. I just wanted to show an example of frequency to the original poster, although it was my personal understanding, not the mean of 10 native Japanese speakers.
I knew it might be absurd, but I wanted to answer to the orignal poster's request.
Therefore, I think #4 is quite reasonable objection or opinion because I was thinking exactly the same thing at the same time.
But I intentionally showed the frequency for the leaner's sake.
And I couldn't help thinking it's interesting that I got just the same reaction here.
Therefore, I think there are two kinds of people in the world: those who like the Showing-percentage-explanation and those who don't.