There's no exact percentage your dld must reach in order for you to preview it: as long as the portion is long enough for the player to actually be able to play something then it's ok.
For example, if you try dlding a file that is 700 Megs big and it reaches 1% then you can preview it ok. However, a 700K file will need to reach most possibly around 10% for you to preview it otherwise the dlded portion would be too small for the player to play or for you to recognize what it is. I mean, how are you supposed to recognize this or that song if you only can hear the first note of it?

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iMac G4 OSX 10.3.9
RAM 256MB
LW 4.10.5 Basic
ADSL anything from 3 to 8Mbps/around 1024kbps

"Raise your can of Beer on high
And seal your fate forever
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The Golden Age Of Leather"
-Blue Öyster Cult-