How do you determine the purchase price in EUR for crypto purchases via USDT?

Hello everyone, how do you actually find out the correct purchase price for crypto if you only have CSV files from the exchange? I often exchanged EUR to USDT and then used USDT to buy various coins. In some cases there were several purchases, sales and reallocations. Now it's not entirely clear to me how I should calculate the real purchase price in EUR per coin. Do you do this manually, with a tax tool or through a tax advisor? Above all, I'm interested in how you get along