You can buy or borrow a scale to weigh your own powder charges to adjust your powder measure.
An adjustable measure is nice to use when working up a load for your gun, then when you find THE load,
make a measure to hold that amount of powder. Making up a WHACK of measures, solves not having
the correct one.
These, made from brass tubing from a hobby store (top row left 2 and top row right 2- with wooden interior plugs) as well as from range - pick-up brass cases run from 20gr. to 165gr.
The brass tubing can be purchased in about any interior size, from 1/8" to 1", in .015" increases - thus, is unlimited.
I have found in testing, that 3/8" interior diameter or smaller, gives much closer "shot to shot" weights of powder. Larger interior diameter measures, including horn or bone,
give large shot to shot "throws".
An old friend, Lester H. Hawkes of Kalispel, my BP shooting mentor from 1972 on, now passed on himself, taught me that consistency in loading is close to 50% of your accuracy. The other 50% is what you are loading AND your abilities.