If anyone knows how to do this it would be a great help!
I recently had a similar problem and couldn't really find a solution for it. The options available to you with PrintParams are somewhat limited.
For page scaling you can choose between: None, Fit, Shrink, Tile All, Tile Large, nUp or Booklet, but you can't specify a specific scaling factor, as far as I can see.
However, if you print a document using such a factor I believe this setting will be saved and then re-used when you print using the PrintParams object. But I'm not 100% sure about that, and it's obviously not an ideal situation...