MAE (Maximum Adverse Excursion) is the difference between your entry price and the worst price the trade reached before closing. For a long position, subtract the lowest price hit during the trade from your entry price; for a short, subtract your entry from the highest price reached.
Example: you buy at $100, the price drops to $95 before recovering to close at $105, so your price MAE is $100 minus $95, which equals $5. Multiply by position size to get the dollar (position) MAE.
You need granular intraday price data covering the life of the trade to capture the true low or high; open and close data alone will not reveal it. Most trade journals, including Tradervue, TraderSync, and TradingDiary Pro, calculate MAE automatically once price data is synced.