I was told to not lose in a trade more than 2% or 3% of your total equity, but such a rule does not consider the effect of drawdowns. If the probability of a drawdown is small, perhaps I can risk more. On the other hand, if the probably is big, I should risk less. Furthermore, is there a threshold for a day? Another question: When I was writing my system, I, to imitate slippage, charged myself $0.01 per share traded. So, a round-trip of a 1000-share position equals 2000 shares traded, or $20 commission. The return was a net $0.008 per share-traded in profit. Now, what I did not realize is that I might be pushing myself to take bigger risks to make up for the high commission. The question is if the profit is so small, why don't I just pick a volatile stock and scalp?