Calculation of phi, derived from the Cumulative distribution function for
N(mean, stdDeviation) normal distribution, given by
1.0 / (1.0 + math.exp(-y * (1.5976 + 0.070566 * y * y)))
where y = (x - mean) / standard_deviation
This is an approximation defined in ฮฒ Mathematics Handbook (Logistic approximation).
Error is 0.00014 at +- 3.16
The calculated value is equivalent to -log10(1 - CDF(y))