I tired googling this, but all the articles I can find depend on you knowing what both the observed and actual values are, and I don't.

So if you are measuring watts, your error is "max error of DMM voltage range" times "max error of DMM amperage range". Very simple. But if you're doing measurements that are a bit more complex, like watts and lumens, or watts and temperature, or anything even more arcane, how would you calculate the total measurement error/uncertainty?

Thanks!