I have a Premium Economy booking to Osaka leaving tomorrow. At check-in, I am seeing the option to upgrade to Business Class for €389, which isn't too bad.

I was wondering how many miles the same upgrade would need. But I am unable to add my Flying Blue number and agent tells me that it would make me lose the Y+ seat assignment if they do it for me.

Is there any other way to see the required amount of miles? Or any recent experience, how Euro convert to miles for upgrades?