I was just about to jump in here pointing out slowsmile's error...but he apparently saw it and deleted the majority of his post.
So back to the original (plugin) question - and in conclusion (I think) -
KF7 % for image width = bad, and putting a percentage for the width (inline or not) that is supposedly calculated for the perfect screen width is useless as it doesn't work. Just put the pixel width/height if it is smaller than the screen.
KF8 % for image width = good, and you certainly don't need to put said percentages as an inline style for KF8 (leave them in the CSS), and said percentages certainly don't need to be calculated to match a specific screen width - you just set 80% when you mean 80%.
So, if the calculated percentage doesn't work on KF7, and you don't need a calculated % for KF8, then what exactly does this plugin DO for those who use Sigil to build ePubs?