Shouldn't need to double post, here's in addition to my comment, since you are not using the "Search Condition" and just not satisfied with how it's boosting results, you will need to set boost values for your normal search as well. Set primary search as boost 2 (^2) then your additional term as boost 0.5 (^0.5), and adjust till it does what you want.
In the end Lucene takes each optional filter and assigns a value to them based on the match, multiplied by the boost value, and adds it up to get your weight.