How do I enable 'Autofill' on my ipad .. I only want to fill up 50%

I do not fully understand your question. "Autofill" on the iPad is a feature that automatically fills in portions of webpages. If you mean reserving storage on your iPad I do not think there is a way.

I think he means for music and movies. once it syncs the content you want it will fill with new content or something. I have really never explored the use of it.

With the AutoFill feature, the iPad can automatically fill in text fields with your personal contact information, usernames, and passwords of websites that you have saved. Safari autofill feature is OFF by default.

How to enable autofill for Safari on iPad

* Tap Settings > Safari and turn "Use Contact Info" On
* Select "My Info" and locate your information.
* Turn "Names and Passwords" On.

If you go to the music tab on itunes with your ipad connected, you can select "Automatically fill free space with songs", but you can not select what percent.

The iPad does not support such a feature, but with iOS4 you have the ability to fill the iPad/iPhone with music that is compressed on the fly to 128kbps… so maybe you can put all your music onto the iPad… or maybe an iPod Classic will be a good christmas gift for your self. With 160 Gigas you are on the good side.

enjoy your magic machine

uuups sorry,
you don't need to wait for iOS4. Just connect the iPad to the computer, then in iTunes select the iPad and in the "Summary Tab" scroll down to the Options section. There you will find the feature "Convert higher bit rate to 128 kbps AAC"

enjoy your magic machine