I was today years old when I realised a 7800 Dura-Ace mech has enough capacity to shift across a 11-40 cassette, AND it works with an ancient 8-speed shifter!

Yeah with a bit of goat link persuasion

How do you check the derailleur hanger alignment?

Or figure out where to set the double B-screws?

There seem to be a massive range of positions and rotations where the derailleur body could be set.