  Let $R$ be a polynomial ring and $I \subset R$ be a perfect ideal of height
two minimally generated by forms of the same degree. We provide a formula for
the multiplicity of the saturated special fiber ring of $I$. Interestingly,
this formula is equal to an elementary symmetric polynomial in terms of the
degrees of the syzygies of $I$. Applying ideas introduced in arXiv:1805.05180,
we obtain the value of the j-multiplicity of $I$ and an effective method for
determining the degree and birationality of rational maps defined by
homogeneous generators of $I$.
