Reply #2 by backlinktalk
Editorial follow-up: why a displayed price is not an execution guarantee
An order book may offer only a small quantity at the price you see. A larger market order can fill at several prices. A limit order controls the worst acceptable execution price, but it may fill only partly or not at all.
In a paper-trading record, write down the order type, assumed fill price and cost assumptions. Avoid awarding yourself a perfect fill merely because a chart briefly touched a level. This makes comparisons more useful without presenting a simulated result as live performance.