Paul - Not sure where the cover-the-BALL in the palm of the hand with powder deal came from - maybe the back woods of Tennesee when he lost the measure that came with his rifle - or - perhaps before the advent of sticken, adjustable powder measures - but - most reading I've done lists powder charges for guns as in grains or drams measure - back to around early 1800's.
Today we use adjustable, stricken measures, or powder scales to 'set' the length of a horn or bone measure to hold the correct about of powder that shoots the best in that gun. Those who know better, know you cannot arbitrarily choose a powder charge and expect the rifle to shoot it's best with that charge - you have to develope the load that shoots best by trying different different patch thickness, different lubes and different powder charges, even different ball diameters as well as different granulations of black powder to find 'the best load' for that rifle, smoothbore, or handgun. I shoot all three with either shot or ball used in the smoothbores. They all require the similar load development. Even different lubes will give different results. Change one ingredient and accuracy changes - same with modern rifles - generally.

As to a handful of powder - that possibly comes from a story by Selous when his 'Kaffer' loaded his 4 bore Dutch smoothbore by dipping his little hand into a 'bag' of powder and dumping in a handful - made a mistake once and put in 2 handsful - Selous paid for that with multiple wounds. Do not try that with a muzzleloading rifle. My mitt probably holds about a 1/4 pound - 1,750gr. - is that a good load - don't think so? We shoot 1/4 pound in a 4 pound cannon. The ball weighs 4 pounds - about a 2 1/2" bore. How 'cupped' do you make your hand?