I can't figure out how to hedge difficulty increases. Maybe I'm doing it wrong.

Here's what I have:

1. August 6 - Difficulty  37,392,766
2. I have purchased miners for 0.14 per Gh/s
3. The table below represents the net income (mined btc - miner cost ) for 6 months of mining, per Gh/s, based on three separate delivery dates for miners (October, November, December) at 5 different possible difficulty increase levels.

BTC Profit per Gh/s at various delivery dates and difficulty increase levels

Difficulty Increase	Oct 1	Nov 1	Dec 1
15%	0.49	0.26	0.13
20%	0.20	0.10	(0.00)
25%	0.09	(0.02)	(0.08)
30%	0.01	(0.09)	(0.12)
35%	(0.06)	(0.11)	(0.13)

Assume, I believe I'll have miners in hand on October 1 and difficulty will increase linearly at 25%.
 
What difficulty futures should I purchase to maximize the mining income? Specifically, at what strike price, and what  quantity.