The compression readings are a bit backward in Shrink. 100% in the right side window is actually no compression applied to the movie. And the more you move down the scale, 90%, 80%, etc., the more compression you get.
So yes, you do want to keep the percentage reading high if possible. The way to do this is by removing certain parts you may not want or need, such as subtitles, or foreign language audio tracks.
Go to dvdshrink.info and go through the action guides set up there. They are a good place to start. There are also some guides here at cdfreaks.