title_body: How much volume of the shot should pre-infusion take up?
upvoted_answer: I was actually reading an article on this yesterday. There is no added volume during pre-infusion. All the pre-infusion step does is wet the espresso fully to prevent channeling.
  
  The article I linked actually shows that less extraction happens during pre-infusion. So to answer your question, it does affect the shot, but not the volume.