Whilst I agree with fathom on regards of the nofollow v follow links, I would guess and this is a pure guess and trying to put myself in mind set of google. In order to spot natural linking v un-natural linking I would expect to see a mixture of follow and nofollow links in a backlink profile. This would be normal for most websites, however if the site only had followed links and lots of them would that damage anything ? I doubt it very much but it may raise a flag as suspect.
Where the 20% rule falls over is when a site posts something that goes viral, many people link to it from many sources including a lot of nofollow sites, would this then harm the site instead of benefiting it, I doubt it very much. I think the OP maybe on about his personal preference and not a hard and fast rule. After all you should not in a ideal world be able to control how users link to you so having 5% or a 85% nofollow I doubt very much if it would have any impact in SERPS.
Just my opinion and pure guess and not backed up with any research.
Good SERPS is great, Traffic however is even better