Well, I'm no dagger-mancer, but AFAIK the formula goes as follows:
Take each poison source you have, and divide its total damage by the duration. This gives you the damage/sec of each poison source. Then take the durations of each of your poison sources and average them. Take the sum of the damage/sec values, and apply that over the average duration of your poison sources.
Say you have Venom that does 200/2 secs, and a PE that does 100/7 secs. This gives you 100+14 = 114 damage per sec, over (2+7)/2 = 4.5 secs, for a total of 513 damage.
Say you add a jewel of envy that does 20 poison damage over 2 secs. Then you would have 100+14+10 = 124 damage per sec, over (2+7+2)/3 = 3.67 secs, for a total of 455 damage.
Say you instead add a flawed emerald that does 20 poison damage over 4 secs. Then you would have 100+14+5 = 119 damage per sec, over (2+7+4)/3 = 4.33 secs, for a total of 515 damage. Here the damage per sec is lower, but the total damage is higher because the duration is longer.
Sometimes getting a high damage per sec comes at a cost of poison duration, as high duration high damage poison charms are not necessarily easy to come by. In these cases, you have to consider how often you will be able to refresh your poison on any given creature, due to factors like targetting other monsters, your to-hit rate, etc. If you will only be able to or only want to refresh infrequently, then try to stick with mostly long duration poison items. If you are willing and able to refresh frequently, you may be able to carry a greater number of low duration poison charms and so forth to boost your damage per sec.