how do I normalise a solr/lucene score?

I am trying to work out how to improve the scoring of solr search results. My application needs to take the score from the solr results and display a number of “stars” depending on how good the result(s) are to the query. 5 Stars = almost/exact down to 0 stars meaning not matching the search very well, e.g. only one element hits. However I am getting scores from 1.4 to 0.8660254 both are returning results that I would give 5 stars to. What I need to do is somehow turn these results in to a percentage so that I can mark these results, with the correct number of stars.

The query that I run that gives me the 1.4 score is:

euallowed:true AND(grade:"2:1")

The query that gives me the 0.8660254 score is:

euallowed:true AND(grade:"2:1" OR grade:"1st")

I've already updated the Similarity so that the tf and idf return 1.0 as I am only interested if a document has a term, not the number of that term in the document. This is what my similarity code looks like:

import org.apache.lucene.search.Similarity;

public class StudentSearchSimilarity extends Similarity {

    @Override
    public float lengthNorm(String fieldName, int numTerms) {
        return (float) (1.0 / Math.sqrt(numTerms));
    }

    @Override
    public float queryNorm(float sumOfSquaredWeights) {

        return (float) (1.0 / Math.sqrt(sumOfSquaredWeights));

    }

    @Override
    public float sloppyFreq(int distance) {
        return 1.0f / (distance + 1);
    }

    @Override
    public float tf(float freq) {
        return (float) 1.0;
    }

    @Override
    public float idf(int docFreq, int numDocs) {

        //return (float) (Math.log(numDocs / (double) (docFreq + 1)) + 1.0);
        return (float)1.0;

    }

    @Override
    public float coord(int overlap, int maxOverlap) {
        return overlap / (float) maxOverlap;
    }
}

So I suppose my questions are:

1. How is the best way of normalising the score so that I can work out how many “stars” to give?

2. Is there another way of scoring the results?

Thanks

Grant