I've recently finished work on a poison damage calculator for Diablo II. After a few revisions, I'm now reasonably confident that it's accurate (or at least, I've corrected all the errors that I've been able to identify myself).
It covers poison damage from both skills and items (and combinations thereof) with +% to Poison Skill Damage, -% to Enemy Poison Resistance, Lower Resist and the target's Poison Resist % and Poison Length Reduced by %. Unfortunately it doesn't really cover duelling, but it can still be used to help calculate poison damage to your opponents.
It's a Microsoft Excel worksheet. I've protected all the cells that don't need any input, and those that do require input are light grey or green. I've made some attempt to make it idiot-proof, but my expertise is limited (e.g. no list or check boxes) and it still operates mostly on the 'garbage in, garbage out' principle. It should (hopefully) be pretty self-explanatory and doesn't require in-depth knowledge (if you know that you can't equip two helms or socket a dagger with 6 perfect Emeralds, you shouldn't have any problems).
If you do have any problems, let me know and I'll see what I can do.
