Now, I've heard of the phrase "rule of thumb" before but always used in the reference of how big a stick a man could use to beat a woman. I do believe that is where the phrase came from, but here's a new one.
From this link: Mosex blog
Sadly, the Princess suffered from what many women today still do – the inability to reach orgasm solely through vaginal intercourse. Defying the social mores of her era, she discovered she could reach orgasm through masturbation. While this led her to blame physiology and not psyche, it still left her deeply frustrated with her husband and eventual four other lovers. But the Princess refused to accept such fate as a permanent condition! Instead, she began some of the most revolutionary work of her time on female sexuality and anatomy while also embarking on her quest for orgasm by penetrative sex.