For tax purposes for "exit price", using end-of-day / closing price should be sufficient. Since this is an ad-hoc, real-time report, you might be generating the report before the end of the day, so maybe using the open price at the beginning of the trading day works better.

As long as it is consistently used is key.

Where an approach doesn't pass muster is where a trader, for example, uses the open price for one trade, and then actual trade price for the next trade, and then maybe closing price for a third trade -- each chosen because it was advantagious for minimizing reported gains. That is what they don't want to see.