  A tight converse bound to channel coding rate in the finite block-length
regime and under AWGN conditions was recently proposed by Polyanskiy, Poor, and
Verdu (PPV). The bound is a generalization of a number of other classical
results, and it was also claimed to be equivalent to Shannon's 1959 cone
packing bound. Unfortunately, its numerical evaluation is troublesome even for
not too large values of the block-length n. In this paper we tackle the
numerical evaluation by compactly expressing the PPV converse bound in terms of
non-central chi-squared distributions, and by evaluating those through a an
integral expression and a corresponding series expansion which exploit a method
proposed by Temme. As a result, a robust evaluation method and new insights on
the bound's asymptotics, as well as new approximate expressions, are given.
