More like 95% >_>
Let's compare it:
[video=youtube;qCUMIDXEC8A]http://www.youtube.com/watch?v=qCUMIDXEC8A[/video]
[video=youtube;IdwloMsROj8]http://www.youtube.com/watch?v=IdwloMsROj8[/video]
0:20 to 0:33 of Terra's theme sounds like 0:57 to 1:09, 1:34 to 1:45 of Fate of the Unknown.
Still a small amount considering the song is over 3 minutes.
Don't exaggerate this so much. Fate of the Unknown is a beautifully composed piece of music and really should be put into the game.