there's nothing predictable about difficulty and calculators offer false reassurance.
Of course, no one can make a 100% accurate prediction, but at least you can have some idea on your mining profitability.
Also, apart from just guessing the future difficulty solely from the historical %increase, miners have spent time to estimate the future hashrate by checking the ASIC pre-order and delivery status of different manufactures.
(FYI:
https://bitcointalk.org/index.php?topic=387533.0)