question about the values in critical_runs array

Hi all,

    in function RGR_FindBestRegression, after get the value of a and b for formula y(estimated)= a + b * x( measured ),
    it counts how many times for every two contiguous sample points cross over the estimated line in n_runs_from_residuals function.

    where are the values in array critical_runs come from ?
    for example, when there are total 50 sample points, if the total runs value is larger than 19, then the value of a and b is reliable.

    the comment is "Critical value for number of runs of residuals with same sign. 5% critical region for now."
    what is the formula or method to get the runs value of 19 for 50 samples?

thanks