I'd always do the mentioned ten drops, instead of one, and divide that by ten to get an average. Try to work your press as if you were loading, ten straight drops with no agitation or powder settling is likely to come out lighter than what ends up in your shells. Once you verify what comes out of your bushing, I think you can use it confidently if you are consistent, it's just a volume measure. When you start changing loads, check all your settings.