I've just been working on a simple JS calculator based on a spreadsheet I created a few days ago:
--> http://xzist.org/amazing-bitcoin-retirement-fund-calculator/

It's not exactly how rpietila describes, it's more aligned to my own personal goals... and I am a complete novice in anything to do with investing.. but I added a 10%, 20% preset rake buttons which I think works how it's supposed to.

only briefly tested, on Firefox and Chrome and *not* IE, tired and going to sleep now, will bugfix and check another day