Also:
"...the negative score may be converted to a positive and applied as additional damage to the automatic hit. Thus, for example, a 20th level fighter with 19 strength and a +5 sword attacking an AC 2 dragon would require a roll of -8 to hit it; the character would be allowed to modify his or her damage roll by +8."
DMG p. 74.
Fighter level 17+ to hit AC 2 requires a 2.
+5 sword is -3.
19 strength is +3/+7.
So, -6 to hit. No damage conversion.
ps- re-reading the tables reminded me of one of my favorite rules; the extra backstab damage. High level thieves could get up to QUINTUPLE damage.
I remember playing with the rule before the retro clones existed, however. Those are typically a consolidation of rules from various sources and I doubt we house ruled the same optional rule appear in those retro clones. The hard part is figuring out that optional rule came from.
Now I'm seeing if I can find it.