mauro: I assume you are working with the "Lucene-Java" package to build your software? (as opposed to one of the other subprojects like Solr, Mahout, or Tika which are the other possibilities that aren't ruled out by your problem description). If so you will probably get more feedback using hte [EMAIL PROTECTED] mailing list in the future.
In general the problem you are describing isn't easily solvable. in order to determine a good "minimum cut off" score you have to be able to normalize your scores in a meaningful way -- to do that you have to be able to define what the "best" (or baseline) possible score for any query is. this isn't something lucene can tell you for any arbitrary query, but it can be determined in special cases. (ie: for a simple TermQuery you can figure it out based on the idf and the document with hte highest tf; for "document similarity" type problems like MoreLIkeThis solves, you can get a good baseline by finding the score for document used to generate the MoreLikeThisQuery (but that requires that it be indexed) -Hoss
