The Tao of Poison is in the "DiabloII.net Links" menu at the top of each page, this one included
It will work out to be around 20 poison damage over 0.4 seconds, not 451 over 0.4.
Each poison source adds to total poison damage, since the bitrates are additive, and so are the durations for ranged attacks. For melee attacks, they're averaged out, so adding a 6 pdsc over 3 seconds with a 451 over 12 seconds will result in roughly a 594 damage over 15 seconds for a ranged attack, or 297 damage over 7.5 seconds for a melee attack.
So poison damage is usually better on a ranged attack, even though the bitrates (damage over time) are the same, the ranged attack's poison will have a duration many times that of a melee attack.