Man, floor to ceiling all the way if you can swing the materials. I did just the top half first in my old 11x13 room and it helped some but the low end was still muddy down near the floor, those corners trap bass at every height. Once I stacked them full height the difference was night and day, kick and bass finally sat in their own space. In a shed that small you're gonna get a lot of bang from those corners so it's worth the extra wool. Only thing I'd say is don't forget the corners behind your listening spot too, not just the front ones.

Kept a 1998 book on the shelf for years and it still saves me on greens. Those inks faded in different ways over time, so what your app reads off a screen almost never lines up with what actually comes off the press. Add in how every phone camera and screen shows color a little differently and you're chasing a moving target. The book gives you one fixed reference that doesn't change no matter what device you're holding. That's why the Denver swatch matched your book and not the app, the app was showing you a guess. Physical books have their own quirks too, but at least they're the SAME quirks every single time.