As @ballardw says, it comes from the t distribution part. I get 12.71 for a two-tailed t value of 0.05. With a standard error of 25, then a half interval is 12.71*25 =317.25, so your confidence bounds around a mean of 25 would look like -292.25 to 342.25. A more precise t value will yield what you are seeing.
SteveDenham
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