Proportion of increased difficulty and increased Hashpower

Hello ...... in the forum I have seen it all, and lots of really interesting posts and links that have helped me to understand, and to adjust the aim of my investments. In fact, I ordered a Jalapeno (April 2013) and thanks to the forum I realized that if and when it ever would come, he would not repaid. and then I applied and got a refund (Thanks to paypal). Now, as they increase the difficulties of mining, try to increase consistent with my chances of spending my hardware. But I am always blown away. Does anyone know how to calculate the increase in Hashpower proportionally to the increase of the difficulty to obtain always the same gain? and if no one has ever created such a calculation table, someone capable, could create the benefit of all? Thank you. Aghos