public string GetValue(Summary summary, BenchmarkCase benchmarkCase, SummaryStyle style) {
  int n = (int)benchmarkCase.Parameters["N"];
  Statistics statistics = summary[benchmarkCase]!.ResultStatistics!;
  double mean = TimeInterval.FromNanoseconds(statistics.Mean).ToSeconds();
  double result = mean / n;
  return string.Create(style.CultureInfo, $"{result:F3} s");
}

double result = ...; double mean = ...; as above
double meanStdDev = TimeInterval.FromNanoseconds(statistics.StandardDeviation).ToSeconds();
double resultStdDev = result * meanStdDev / mean;
return string.Create(style.CultureInfo, $"{resultStdDev:F3} s");