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Incorporating real-world parameters into simulated(paper) trading
You said the decision will be made by EOD. If you've made the decision prior to the market close, I'd execute on the closing price.  If you are trading stocks with any decent volume, I'd not worry about the liquidity.  If your strategy's profits are so small that your gains are significantly impacted by say, the bid/ask spread (a penny or less for liquid stocks) I'd rethink the approach. You'll find the difference between the market open and prior night close is far greater than the normal bid/ask.