In AD&D i have always used this rule. Today i want to share it, it is rare stuff because it comes from "Australian Realms", issue 13 (here),one of the most elusive fantasy magazines around (good luck to find those issues on ebay).
But i don't want to store this treasure jealously, so here it is. To me, this is bible. The philosophical thought of Mr. Paul Mitting is the truth, concerning this topic.