I tried the line on the back of the derailleur as well and it doesn't matter if the line is above, below, or right at the teeth tips, the line isn't perpendicular and one side angles away, like the angle is wrong. I cut the chain at front and biggest rear plus 5 and a quick link. Something seems off and not finding what and the shifting isn't great and isn't spinning very smooth.
I've gone over Shimanos installation documents a bunch and keep double checking and expecting to find something off but it hasn't happened yet...