A good way to set your powder measure for light charges is to throw 10 charges, weigh it, and divide by 10 to get your average charge.
If you want 3 grains, get the powder measure close, then throw 10 charges and if it weighs somewhere close to 30 grains, you’re probably good to go.
I believe you’ll get as good if not better accuracy with similar volumes than similar weights.
You shouldn’t have to weigh every charge. An occasional check should suffice.
