Dear Bingley: just for the fun of it I tried to see if I could find out when that "rule" came into use. No luck,so far. But I found this, which it amuses me to quibble with:
Adjective Comparative Form Superlative Form
hot hotter hottest
ripe riper ripest
creamy creamier creamiest
spicy spicier spiciest
fragrant more fragrant most fragrant
well-done more well-done most well-done
flavorful more flavorful most flavorful

Der Quibblemeister asserts there is no comparative of
"well done". More than well-done would be "over done".
If you're talking about roast beef, you would do better
to ask for it by color, red, pink, and gray.