how is DoT affected by buffs?

for example, if i pop 20% dmg increase, then use a DoT, will that DoT do 20% more damage, or will the ticks during the dmg increase duration do 20% more damage.

or, if i DoT, then do a dmg increase, will that affect it?

same question for crit increase
basically for the crit, should i ALWAYS have Straight Shot up first before Venom/windbite?