Konstantinos,
First of all... the passing score is no longer 61%. That is an old number and unfortunately still appears in some documentation (even ours and we're working hard to delete it....)
Please read this article to learn more about the passing score:
www.deepfriedbrainproject.com/2009/12/po...exam-pass-score.html
Once you have read this article and understand how the passing score is actually calculated, you will understand that I cannot really answer your question. 80% is really a "best practice" that many PMP trainers support. If you feel that this is too high a safety margin, then please feel free to lower it for your own calculations. But as a PMP trainer I have to "insist" on the 80% margin (just to be safe).