I bring a handful of .38 special casings every time I shoot BP. It's a great quick 'n' dirty way of getting a fairly close consistency in performance.
I also have a couple scoops made out of a .357 and some tie wire. However, if the wind allows it, I like to use six .38 casings. I fill all of them up to the level. Then I load each cylinder with a charge. I do this so that I don't lose count or double load. I know it sounds stupid, but I have been bringing novice shooters with me to the range the last couple of times. They usually wait until I have my hands full to ask a question
By loading each chamber with a single case, I know when I have gotten all of them charged, and it allows me to visually inspect each one to make sure the powder is seating in there correctly with the wad over it.
But when I don't feel like going through all that, I just use the .357 scoop and get "close enough"