Hello,
as I recently invested in a PBMining contract I've create a simple profitability calculator. It allows you to balance your investment based on your difficulty increase rate estimation. Also it allows you to take into account re-investments of your earnings at a given rate (0-100%).
Check it out & I would be happy to get some feedback on the functionality:
http://pratscher.org/pbmining/Thanks!
According to this if you reinvest any part of your earnings you will be at a loss, unless you are very optimistic about the difficulty increase and the price drops from PB.
Well, it depends at which point you stop your re-investment. If the difficulty raises faster than the price decrease from PBMining you will make a loss, otherwise a profit.
I also added the functionality to stop re-investment after a certain period. This allows one to find the best time to stop re-investing money and cash out.