For the time that I have, the best bang for my buck has been posting on .edu sites. They are LOVED by Google, and usually have high PR. Although tough to get approved, when you do, they are golden.
For the rest of my time I have found that I get the excellent results from backlinks that I have posted on blogs that are relevant to my topic(s). Make sure the blog is relevant to their site, and you will have a much higher success rate of getting approved.
I am not a fan of article sites...the amount of time that has to be spent first writing, and then going through the process of getting an article approved (ezine is the WORST), I find that time doesn't pay off nearly as much as creating my own relevant AND thoughtful blog postings. What ever you do, don't just write something stupid like "Hey I like your blog." Chances are, that wont' get approved.
As far as your 80/20 rule...create a list of which pages you want to link back to, make the posts, and in a few weeks use something like "Rankerizer" to find out how it's working.
