I found in this article the formula that MySQL use to ranking in FTS
w = (log(dtf)+1)/sumdtf * U/(1+0.0115*U) * log((N-nf)/nf)
dtf is the number of times the term appears in the document sumdtf is the sum of (log(dtf)+1)'s for all terms in the same document U
is the number of Unique terms in the document N is the total number of documents nf is the number of documents that contain the term
But I wonder that what is 0.0115?