"N days since last finished"
This may be the last straw that makes me install OmniFocus on MacOS as a VirtualBox guest on Windows!!Yep--OmniFocus has this. I got used to it and was indignant that Reminders on the Mac doesn't have it.
Unfortunately, OmniFocus isn't available under Windows. (Though you could get the iPhone version.)
Thanks! Or, rather, unthanks - since I will probably waste a lot of time on that. :-(