boosting based on number of terms matched?

[DarkNovaNick]

I'm using the edismax handler, although my question is probably the same  
for dismax. When the user types a long query, I use the "mm" parameter so  
that only 75% of terms need to match. This works fine, however, sometimes  
documents that only match 75% of the terms show up higher in my results  
than documents that match 100%. I'd like to set a boost so that documents  
that match 100% will be much more likely to be put ahead of documents that  
only match 75%. Can anyone give me a pointer of how to do this? Thanks,

Nick