For a given document, Solr may determine the interesting terms and their weights:
"interestingTerms":
["field_b:foo",5.0,"field_b:bar",2.9085307,"field_b:baz",1.67070794]
which can be used to generate the following search query:
field_b:foo^5.0 field_b:bar^2.9085307 field_b:baz^1.67070794
So MLT is AFAIK a two step process that finds the interesting terms and weights of a given document and then uses those terms to do a search
See https://stackoverflow.com/a/12328229/604511 and mlt.interestingTerms in http://wiki.apache.org/solr/MoreLikeThisHandler .
Do you have a good reason for such a threshold? Just present the results to the user. If there is low similarity, the user will (and must be allowed to) overlook the results.
See the following: StackOverflow concentrates on the why does
and fetches nothing about tomcat. But still SO users overlook bad MLT suggestions all the time.