Suppose I am short and would like to set a stop to be triggered iff the Mt. Gox price rises above $3. If I expect a $0.1 spread on Bitcoinica, where should I set my stop? $2.95, $3.00, $3.05, or.... ?
It's a mistake to think in terms of Mt. Gox price, because the spread is not static, it changes with trade volume and other factors. You should think only in terms of the bid/ask prices you are comfortable with, because those are at least real values that can affect your trades.