Poison build

Hello,
I was trying to build something with poison damage on my rogue and i had some questions on how things interact. For example, there is a node in the hail of arrows skill that transforms the skill from Physical to posion. How does that interact with Acidfletch (bow that converts 100% of the rain of arrow damage to poison).
Also … if i add a lot of phys damage to my build when is the damage converted? Before the multipliers or after?