Hi everyone, I have been playing xerath for a while now and he's one of my favorite champions. But, I have one question about spell penetration. For xerath, I wouldn't get the spell pen boots but instead the CD reduction boots. That being said, I would have 0 actual spell pen and would have sometimes over 50% spell pen. With having actually 0 overall spell pen, Would my W and buying void staff be utterly pointless for the spell pen %? I have had different answers on this such as spell pen percent is its own stat. Can anyone help answer this question?

Try reading this:
http://leagueoflegends.wikia.com/wiki/Magic_penetration

Flat and percentage magic penetration are applies differently, with flat being applied first.