I started out loading from a flask because a lot of pictures showed it being done that way. I never had a problem. I always use a measure with a rifle. One day I asked myself what is the difference? After that I no longer loaded from a flask. I shoot a 1851 Colt Navy Replica, 36 caliber. I use 18 grains of FFFg black powder, weighed. DO NOT weigh a black powder substitute as it is a volumetric only equivalent and as I understand matters weights of the substitute can vary from various production runs. In weighing the charges I feel I get the best accuracy. I use a wonder wad and round ball. The balls are seated about 1/16" below the end of the cylinder and I put a mark on the ram to insure even seating. From a rest I can get about an inch and a half groups, MOST the time at twentyfive yards if shooting from a rest. This is about as good as a lot of modern day pistols.
I have made up a batch of paper tubes with covers, each tube holds one powder charge. You could probably use old cartridge cases and corks. If I want to shoot fifty rounds, I fill fifty tubes. The tubes can be reused. I store them in a plastic box and use tissue paper to keep them from moving around. Good luck.