If you can access your account in bittrex, you can obtain your exact history, and if you do not know how to extract your trading list, contact the support team (some platforms have limits of six months to a year, after which you will not be able to see the past)
If you can't log in, type an estimate and tick that when you want to calculate taxes.
This is not tax advice, and may vary by country.
I have access to my bittrex account. but what do you mean by "exact history"? you mean my order history? if yes, I have access to that too. but what I want to know is, how can I backtrace my deposits to my bittrex account? example: I deposited 1 ETH to bittrex but I dont know to which price I bought that 1ETH before depositing.