[vonneumannstan]

>What I'd really like (and haven't seen so far, but also haven't searched too hard) is the derivation of an error function that tells me how wrong I am to treat things classically, depending on some parameters (like number of particles, total mass, interaction strength, temperature, whatever is relevant).

There are plenty of ways to do this and things like Wigner functions literally calculate quantum corrections to classical systems.

But generally if you can't even measure a system before it's quantum state decoheres then it's quantum status is pretty irrelevant.

I.e. the time it takes for a 1 micrometer wide piece of dust to decohere is ~10^-31 s and it takes a photon ~10^12s to cross it's diameter. So it decoheres 10 billion billion times faster that a photon could even cross it.