Never touched the inner surface? How'd you miss that?

Switching to measured ratios changed my whole approach too, and once you get comfy with it you can tweak the developer for just your roots instead of frying everything to match. For fine hair I'd start lower on the volume and go a touch darker at the root, then gloss the ends so it doesn't grab too much. It sounds like you already figured out the biggest thing, which is that box dye can't adapt to your hair the way you can.

Man, that false reading stuff is the worst (had a similar thing happen with a dead battery in my tester once) and it always seems to happen right when you're already frustrated. Glad your buddy's cheap meter caught it before you tore into the drywall.