As noted in the NLEstimate macro description of the DF= option: "The degrees of freedom for testing a linear combination of parameters in a linear model would typically be the number of observations used in fitting the model minus the number of parameters estimated in the model – essentially, the error degrees of freedom." So for your example with 120 observations and a single binary predictor resulting in two model parameters being estimated, you could use 118 degrees of freedom. But note that large sample Wald statistics are often used, as they are in procedures like GENMOD, which are chi-square with 1 df. So, you could get the Wald statistic as (estimate/stderr)**2 and its p-value is 1-probchi((estimate/stderr)**2, 1) . At df as large as 118, you probably won't see much difference in the two p-values.
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