Swifty Morgan said:
I wasn't shooting for any weight in particular. I was trying to get the measure working consistently. The actual charge I needed today was 10.0. I thought there was no point in chasing 10.0 until I had some idea whether the measure was working correctly.
Maybe I'm off base but, IMHO, if you don't know what charge weight you're looking for, you can't assess the performance of the measure.
Tolerances can be expressed three ways: You always start with the preferred measurement, whether that's a dimension, a weight, or a volume. From there you can have the tolerance expressed as plus-or-minus (the most usual), plus-only, or minus-only.
If your goal is 12.5 grains and you get weights ranging from 12.4 to 12.6, that's a tolerance of plus-or-minus 0.1 grains. Not too bad.
But if your goal is 12.4 grains and you consistently get loads from 12.4 to 12.6, the error is always on the high side, by as much as .2 grains. If you're sneaking up on a maximum load, consistently throwing more than what you want is not a good thing.
Refer to stinkeypete's post above on the distinction between "accuracy" and "precision." If you have a recipe for a maximum load of 12.5 grains and the measure consistently throws 12.6 -- well, that's very precise (repeatable), but it's also consistently over the maximum load, so that's not very helpful.