I do not risk a set amount per trade. In fact, my risk is open ended because I do not know how many positions I will have and I do not use stops. I just trade what I get and let the market tell me when I'm wrong.
I determine my position size based on my best estimate of what a "really bad day" would equate to in dollar terms. This is just based on my experience and 5 years of detailed statistics.
Because this is such a capital intensive strategy, it is difficult to trade in a retail account. In my opinion percentage returns in a prop account are meaningless because of the amount of leverage that is in use at times. I only think in dollar terms, both for what I earn and what I risk. These dollar terms are only relative to how much risk I am comfortable taking and how much of a cushion I have with my account equity.
The way I trade this strategy, it's been very consistent for me. You can get an idea of my max drawdown from the equity curve I posted earlier in this thread. 2006 was the least profitable and most inconsistent this strategy has been for me in 5 years. I basically flatlined for the first 2/3rds of the year.
The point that I always emphasize about opening orders is that it is not really a "strategy" per se. A trading strategy involves other elements like risk management and exits. The discussions in this forum revolve around the "getting in", participating in the opening print. After you are in though, you are left to your own devices. The street is littered with stories of guys who have tried this strategy and lost money or lost their patience. It's not for everyone. It is just not going to be a good match for lots of people's trading personalities.
You still have to know how to trade.