Eg, if I'm trying to optimise for the term "blue widgets", I understand that to look natural, some links should say "red and blue widgets", "widgets that are blue", "widgets", "cheap blue widgets", etc. What I'm unsure of is roughly what percentage should ideally have "mydomain" or "mydomain.com" or even just "visit this site" -- ie have no targeted anchor text at all.
I did read somewhere that someone who had a number one spot had about 82% of the inlinks saying the exact keyword phrase. The trouble is that results can vary, becuase links all have different values, one pagerank 5 link could have the same value of 3,000 pagerank 1 links. I would if possible get as many "blue widgets" as you can, becuase you always come across link exchanges, directories etc where you can't use the anchor text as you would like to, so it evens it out.
I've been experimenting with this for several years and have found that 75% targeted anchor text and 25% varied anchor text has worked perfectly for me