I've been using gentoo for a decade and I'm sad to say that I still can't get to 100% built from source. There is always some useful program out there, or something that I don't want to maintain the build process for, that ends up being a binary that I know nothing about.
Getting to 90% is possible. 100% is extremely difficult requires making some hard choices about what software you are going to use.
0. http://sobersecurity.blogspot.co.uk/2016/05/trusting-trustin...