Sorry once again for making it all about esprit 100's, but I just wanted to quickly say that I've spent a lot of time making microadjustments to my backfocal length and I'm actually very surprised at the flatness I ended up getting. I was actually out by a couple of mm. It's not perfect but a LOT better than I had it before. That's on an APS-C sized ASI071MC.
So, yeah, everyone's right...the esprit's flatter than I gave it credit for.
https://imgur.com/a/zA4LWtd
