I probably should pipe up on this one since the concept is originally mine... but I never could get the math right and eventually decided not to bother, so I'm glad you've made some effort, Jedi.
The concept with this is simple: if you're using some monitor distance, it's always going to be wrong everywhere but one place, so let's just minimise the error overall, right? But it's not so simple as splitting your 0% and 100% distances (averaging) because it's nonlinear. That's where the math gets fun. It's not totally hard math but it's not garden variety either
Then it gets saucy when you realise there's a difference between minimising error magnitude (how wrong is it, 1 dimension) and error volume (how much of the screen is wrong and how wrong is it? 3 dimensions....)
Reason I decided not to bother in the end was because, put simply, the more I brushed up on my calculus (it's been a long while) the more I realised that 0% was 'the way'.
Buuuuut, back then I was completely focussed on in-game matching, and cared not for the desktop. And I had the same experience as you, jedi: I'd leave game from flicking to heads like it was easy and I couldn't flick to an OK button or a text box if my life depended on it. And I don't know about the rest of you, but I use the windows desktop a LOT. More than I do 3D gaming, such is life. As far as I can see it, if there's any value to maintaining sensitivity between games and zoom levels, there is that same value in maintaining it to the desktop.
Trouble is there's absolutely no way you're matching the desktop to any more than one FOV. So you've definitely got to pick one. I tend to play the same kind of games so my hipfire FOV is always the same and it's not an issue for me: match desktop to my standard hipfire, then 0% from there in the 3D world. But for people who don't have a standard hipfire, I'm not sure there's any way to extract any benefit in matching to desktop at all.
And then there's the million dollar question of what defines desktop speed in the first place. 0%? 100%? Horizontal? Vertical? Fight!
And the very real consideration of how far we are from the monitor/monitor dimensions, which matter less in the 3D space, but actually help to answer the above question, in a 2D space.
Welcome to my years-old rotting can of worms.