Need help figuring out what to max on updated swain

Most of the people are saying to max his skill order in R>Q>W>E but I think maxing E second is better since its a damage over time spell instead of the flat damage on W. I'm not sure how to do the math figure to out which maximizes his damage but I imagine his single target damage would better with E maxed second. Would like get a response from someone who could do the math while taking into account AP,AP ratios, and his 20% damage amp since I'm not sure how. Please and thank you