Actually, regardless of what your THAC0 or levels are, the chance to roll a Critical Miss is always 5%.
I once had a screenshot of Korgan getting three Critical Misses
in a row on the
unconscious Guard Captain in Galvena's Festhall. Now
that's just sad. He had an audience, too: Nalia was there, watching. Way to impress the ladies with your combat abilities, Korgan.