I suppose this is a question asked frequently.
I think never. Pay with credit card or ask recipient for EUR bank account. But if you still want to do your way, then you have to find website of recipient's bank and see what is his exchange rate and guess the value for the day it will reach this bank. (SEPA can last even 3 days, or more if from Friday to Monday). Don't forget about including all fees. Not all banks in Poland accept SEPA transfers for free.