Make more specific

Hello, I am using this module to display synonyms on a taxonomy driven biology site. I have nodes called species which contain a hierarchical select taxonomy to assign a taxonomy to the species, and I also have a node called synonyms which represents other things that the species has been called, but the names are no longer valid. Within the synonyms node I have a hierarchical select taxonomy for the synonyms and the valid name. I would like similar_by_terms only to display similar nodes (synonyms) on the species node when the valid name taxonomy in the synonym node exactly matches the species taxonomy in the species node.

At the minute it displays the synonym on any species page where the taxonomy contains any similar terms. Obviously this is what it's meant to do, but I would like to fine tune the specificity.