  Fix a square-free monomial $m \in S = \mathbb{K}[x_1,\ldots,x_n]$. The
square-free principal Borel ideal generated by $m$, denoted ${\rm sfBorel}(m)$,
is the ideal generated by all the square-free monomials that can be obtained
via Borel moves from the monomial $m$. We give upper and lower bounds for the
Waldschmidt constant of ${\rm sfBorel}(m)$ in terms of the support of $m$, and
in some cases, exact values. For any rational $\frac{a}{b} \geq 1$, we show
that there exists a square-free principal Borel ideal with Waldschmidt constant
equal to $\frac{a}{b}$.
