Thank you, Dad. I can see patterns in Mt. Gox charts but can not short nor set stops on Mt. Gox. I would like to understand Bitcoinica's behavior. Is it executing my BUY-STOP when the BID is greater than my BUY-STOP, or the BID/ASK mean, or what?
$3.05 STOP for a Mt. Gox $3 and $0.1 Bitcoinica spread were useful answers, but I'd prefer it confirmed by Zhou.
I can't really promise anything on the spread because it's set by an algorithm. There's some human factor behind it to adjust the market risk that we tolerate (the guaranteed liquidity we provide), but basically the spread mostly varies with market conditions.
According to my estimate, if there's a wall (at least 100 - 1,000 BTC) at $3.0000 and we're going to break that, the Bitcoinica price will be between $3.0087 and $3.0237. Therefore, if you set a stop at $3.05 and it's triggered, it's either the wall has already been broken, or the remaining of the wall is very insignificant (e.g. 80 BTC).
$3.05 should be fine in your case. Even though I personally confirmed this, this is not a guarantee. It's just the experience of most traders. You always have to take risk to strategize your trades.