Could someone please explain, how to calculate a fee more or less exactly to neither over- nor underpay?
Just see here
https://bitcoinfees.earn.com/ . It show sat/byte. So, just check needed fee for you and then type into necessary field. That's all. System will make everything other by itself.
To reasonably use this I need to know a size of a transaction. But I can get it only after I press the button "Send", so after a fee is already set and the transaction is out of my control.