It depends on the BP Firearm and what I am trying to accomplish ... figuring a charge for accuracy, plinking, Hunting , with a Revolver,, in the field or at a bench, a rifle , a Shotgun... ?
Revolvers in the field or at the bench I load from a measured spout on a flask... calculating a load for a particular I'd use a incremented volumetric measurer. Rifles a powder horn and a predetermined measurer ... a Zouave .58 Cal Rifled Musket I use a Zouave Flask with a 60gr spout...or Paper/linen Cartridges.
That's how I do it, I'm not tellin' a single other soul how they should do it... but it's been workin' well for me for 30 years with no hand grenade Flasks...
Hope that's what you were lookin' for.