Yes, yes and yes. That is the ONLY way to measure and pour safely. Never charge directly from horn or flask.
Quite Right!
So what you do is you get an adjustable measure, and you work up a load that is accurate from your rifle to the max range at which you decide you will shoot.
THEN
if you wish..., you fashion a fixed measure, as Pete G mentioned, a "
charger" that always gives you your correct load. That way you don't have to remember which rifle likes which granulation of powder and how much..., you just use the charger, and a lot of folks mark them thus
70 gr 2F or
80 gr 3F ...etc. Some folks don't mark them and simply set up a horn and measure for each rifle or gun.
The reason why you don't use the whole horn like they did in the 1960's on TV is that ever present, though slight, chance that there is a smoldering ember in the barrel....so you pour the premeasured charge of powder from a charger or an adjustable measure, and if it does "cook off" well, you have some singed finger tips. IF you're using the whole horn..., that stream of powder ignites and travels up into the horn, and you're holding a
hand grenade, and you're likely losing some fingers or worse.
The valve if you have a fancy horn...it's usually brass, so you're risking your fingers or worse , while betting dollar's worth of brass is going to protect you ???
So as the man wrote, you always use a charger, and you don't run anywhere near the risk.
LD