How much of a scenario do you ought to know (when you start playing it)?

[FLG]

I try to play them blind but there are not many which I have not played or playtested over the years.

I will say I have slaughtered the opposition a few times in campaigns when I have moved whole corps or armies onto a new axis of attack. Having too much knowledge of the starting positions can be a big draw back if your opponent is sly and you don't deploy your forces in line with good practice. Play what is in front of you, keep your forces in order and don't over expose yourself, even if you think you know what is coming at you.