It turned out that it was a very simple calculation to compute $\langle \square,\square+2\rho\rangle$ for $sl(n)$ using Cartan matrices. For $sl(n)$, $\langle \square,\square+2\rho\rangle = 2-\frac{1}{N}$$\langle \square,\square+2\rho\rangle = 2-\frac{1}{n}$, and hence $$t_{\square\square} = q^{2-\frac{1}{N}}.$$$$t_{\square\square} = q^{2-\frac{1}{n}}.$$ Note, by the way, that the convention used here is $q=e^{\frac{\pi i}{k+h^\vee}}$, where $k$ is the bare level, and $h^\vee$ is the dual Coxeter number, which is just $n$ for $sl(n)$.