From reading the tooltip, as well as this post
http://forums.nodiatis.com/forums/sh...ad.php_t=15897 one would assume that Shaft combines the damage of both burns and poisons before registering whether it will proc. Just curious if this is the case, or if it calculates separately and would require for example 10k poison damage to shaft an enemy with 10k hp even though they have 4k burn damage as well.