to ascertain the true rise/fall in bitcoin difficulty, you don't need a calculator, you need a mind capable of prophesy.
There are so many factors which could make difficulty rise as well as fall. what if people realised that their 130nm rigs were no longer paying as much as they were costing - funny thing - it's time to switch off for lots of folks, but nobody does!?
what if the KnC datorhall fell through a crack in the earth (that would be nice)
what if?
what if?
what if?
ask me again in ~8 days and i'll give you as close an answer as I can to what the next difficulty change will be. That's the best anyone can do. get over it... move on...  anyone running old 130nm rigs, and criticising PBMining that they won't make break-even are just as guilty of maintaining idiosyncrasies as those who don't understand - DON'T PAY OUT MORE THAN YOU CAN AFFORD TO LOSE - Cloud Hashing, or Mining Rig.