Valid Domain

To achieve a relative accuracy of $10^{-3}$ or better, the valid domain is restricted by equations 18 and 49. That is, $Z>4.8$ (or $\log(p) < -14.05$) and in practice $F > 1$ for all degrees of freedom (or $\log(p) < -1$) when using 20 iterations. This means that the overall $F$ to $Z$ transform ($Z(F,D_1,D_2)$) will be valid for $\log(p) < -14.05$.