Never heard that 30% rule before. Where is that documnted at Google? More internet myth, semi logical but not correct.
I've seen word for word articles indexed in the top 10 by google. Only difference between them was the titles and descriptions, body text was 100% the same. Lol, it's another good reason not to let google pick your description.

I've seen the same thing with articles I've submitted to many of the articles places. Sometimes two or three of them will make top ten. Again, way less than 30% difference but for sure different titles and descriptions.
If the 30% rules was true billions of documents on the web would never be indexed be cause they would be kicked for being duplicate content.
If I were ever to put two of the same documents on the same site I'd for sure want to make sure they were at least 30% different and I can think of cases where I might do that. That would guarantee both documents would get indexed and rank but I've never seen it make much difference on seperate sites on seperate IP addresses.
Post the exact same article on a blog and on a plain vanilla html webpage and they aren't even going to be close to the same to google. Duplicate content gets indexed all the time, all over the place. Lol, sometimes just changing all the it is to it's and it's not to it isn't and some other extra wordiness to contractions is enough to rank higher than the exact same article.
My experience says there is no 30% rule for dupe content, nor even a 5% rule. Unique is what google wants and unique is what we bith give them. You're omfortable with 30 and I know that works, I'm comfortable with a para or two and I know that works to. Lots of ways to do things on the net and a lot of the ways work for the people doing them.