normal distribution calculator
There is no analytical solution to the cumulative normal distribution. NormDis uses Hastings' numerical approximation as described in this Wikipedia article in the section "Numerical approximations for the normal CDF" referring to Zelen & Severo (1964).
PROBABILITY OF EXCEEDANCE
The exceedance probability (EP), being the probability that X is greater than Xr equals 1 - CP. Hence, when CP is known, the EP is also known.