https://www.parktool.com/...gnment-gauge-dag-2-2

Bike tools are great, but this is the only tool in my arsenal that isn't duplicated by some other tool, and also has the magic effect of fixing like 90-100% of shifting problems with a single adjustment.

You will drive yourself insane trying to fix slightly errant shifting on a bike with a only slightly bent RD. It'll be 'just slightly off', and you'll think a little barrel adjusting here, a little cable tension here, maybe better cable routing here, etc. - none of it will help. Then you'll align the hangar, and voila - everything is PERFECT. No adjustments needed.