Now I wonder how do I determine my selling and entry prices in fiat currency? I don't know where to find the excact bitcoin or BNB to dollar values for the excact minute of my trade.
To make it easier why not used stablecoin for pairing so you can count the fees or less on the trade. If you used busd on binance there is no fee so there is no deducted during the execution of your trade, while for pancakeswap they incurred fees. Thats normally visible during the end part of the transaction informing you that they will less that amount. Not sure if this is what you meant, or tax in real life trades.