Jacob: in my experience, pouring powder from a horn into a measure, then from the measure into the muzzle is as consistent as is required to shoot one hole groups offhand at 25 and 50 meters, and slightly over a minute of angle at 100 meters. Ned Roberts, in his book "The Muzzle Loading Caplock Rifle" describes the stricken measure as being the most consistent. That is filling the measure from the flask or horn to overflowing, then striking off the excess with a piece of horn or wood, or even a knife blade.
As has been mentioned, spend a half hour in your shop with a scale and your horn and measure, pour charges, weigh them, dump them into a vesssel, then repeat, to see how uniform you can throw charges.
There is no need to tap on the measure to settle the charge. If you need that extra 2 grains in the charge, make the charge bigger and just use the pour and dump system.
My brother goes so far as to proclaim that using a scale to thrown charges even in a cartridge like the 45-70 as an example, will not produce better consistency on the target than weighing every charge to the tenth of a grain. And he beats me regularly at that game.
And I agree with Pukka Bandook, that black powder is not particularly fussy about precise charges.
Don't make yourself crazy over this by overthinking it.
