Smith and Weintraub used a value of $n_0-1 = 288.04\times10^{-6}$ which was the average of 3 published values for visible, 9 GHz, and 24 GHz electromagnetic radiation measured at $T_0 = 273\,\textrm{K}$ and $P_0 = 1013.15\,\textrm{mb}$, which gives

$$n_{1} = n - 1 = 77.6\times 10^{-6} \frac{P}{T}$$

(I figured this out while wrestling with a similar commonly-used but obscurely-sourced equation relevant to my own question "How distant is the horizon on Venus?".)