Bro, forget 20%, I used less than 5% (and never exact match)!! And I used unique articles each time for all my WEB 2.0's and I linked out to authority sites, put images, videos and made all the WEB 2.0's manually by hand! Each even had a detailed "About Us" and "Contact Us" page. I reached the first page and then got slammed by Penguin 2.0. And this happened for 2 different clients of mine. One local and one nationwide. I analyzed Ahrefs and Majestic and both showed the target keyword with an anchor text ratio of less than 2%!! So yes, Google definitely has a way of finding out. The only site that survived was a highly authoritative website that received a ton of natural links on a daily basis. What does that tell me? It means that if you have a new-ish website and have 50 WEB 2.0s pointing to it, well guess what, its pretty obvious what you're doing. On the other hand, if you have a website that already has hundreds and thousands of backlinks and you point some smartly created WEB 2.0's, its not a big deal. I mean think about it, how natural is it for a BRAND NEW website to have WEB 2.0's pointing to it? Extremely unlikely.
Bottom line? Do not start pointing WEB 2.0's on brand new websites. Start with the more natural looking links (like local directories etc). Build some basic authority and then MIX in some WEB 2.0's.
Just my 2 cents from experience.