Any prediction on "future difficulty" is just a guess - there is no way to know much in advance when new hardware will appear, how much will be available, how much older hardware might go offline due to unprofitability vs how much come back ON line after a price spike/rise, etc.
With that said, IMO BitCoinWisdom is probably overall the best combination of a calculator (that you can configure most parameters on with YOUR guesses and YOUR costs) and a history of hashrate/difficulty and access to most of the major pool pricing information to BASE your guesses on.
Thanks. I'll look at the bitcoin wisdom site and see how much of a rise in difficulty will make my investment worthwhile.
I think that this is virtually un-knowable. I am sure it will be much higher. In twelve months I expect it will be at least 30% higher than it is today. A higher BTC price always ramps up the hashrate as people buy more gear, and "marginal" gear continues to be run.
Just my $.02 on the subject.
All predictions are guesses they're not often certain. I was hoping to find a high estimate/low estimate. Although, if you are close with the difficulty prediction. I'll ROI on my investment in time.