Zend_Search_Lucene syntax error exception when searching on "to"

One other question. Is there an upper limit to the search
hit score? 
Does a score of 1 mean a perfect match? The reason I ask is
that if it 
wanted to display a five star rating for each search results
relevancy, 
what should the score threshold for five stars be? Are there
any 
commonly used figures for something like this?

Thanks,
-- 
Jack