Hmmm.... I'm not so sure about comparing the "Unique Visitors" metric to "New" or "Repeat" visitors. "Unique Visitors" is a way that SiteCatalyst/Adobe Analytics "de-dupes" visitors who may have visited your site multiple times during a certain interval of time and counts them only once. I don't think it has anything to do with whether or not they were a New Visitor (visitor id never seen before) or Repeat Visitor.
Back to the original question, I am not sure we can get SiteCatalyst to do this one for us internally. I think you'll need to do it in a spreadsheet with data you get out of SC. For any period of time, I'd get your first time visitors visit count from Visitor Retention > Visit Number and divide that by the total number of visits for all visitors.
Shown below is a copy of the Visit Number report to give you an idea.... please let us how it turns out!