Originally Posted by mnpaddler
..an empty milk container as a measure for a gallon and it comes out 1/3 of a cup short...
I'm sure my precision leaves something to be desired, but I pretty much don't measure anything in my batches that precisely. I prefer the RDWHAHB approach. If you're into anal precision (I understand, I am about somethings) a gallon contains 16 cups, so being a third of a cup short's not likely to ruin a batch of your current project (it's only a little over 2% off).
As to the recommendations about weighing, buying other items I'd say screw that. The point of measuring is to obtain known quantities. If your milk jug is 1/3 cup short, add the difference to your pot/carboy etc. I couldn't see buying something (especially not dragging out a scale and temperature conversion matrix chart!) when the FREE jug on hand does the job.
Your mileage may vary, but I'd suggest simply relaxing about it. Add the excess manually if it worries you. Or buy something more precise if you've got the money to spend.
I've never calibrated my equipment because for what I've done so far, "ball park" is close enough and has allowed me to replicate (at least to my amateur palate) what I've tried to.