depends on the plane, really...
but generally, I like 75%...
50% will give you a view closely approximating what you would actually see... with peripheral vision included... but since your monitor exists only in front of you, not to the sides, I fly 75% and limit my field of view to what is directly ahead - in that way, 75% is more realistic...
experience and personal opinion, mostly... by the same token, I think that on most planes, 100% zoom is rather too close...You're saying what I've read. 50% is realistic. How do you know that? Is it somewhere in an fs faq or something?