Not all the powder would burn but would be found laying on the paper. You keep reducing the powder charge until you stop getting unburned powder on the paper.Then you add or subtract tweaking the charge until you get the accuracy you wanted.
That's a good sound piece of advice kraigwy. I really like hearing about those old timer ways. Would have made one very interestingly and fun book to read no doubt if all those quips & suggestions were written down back in the days of real traditional shooting
. A time when there wasn't Triple 777, Pyrodex RS-P or Select, poly-patches or Designer Smelling patch lubes.
OP not wanting to be a naysayer but a suggestion. Be careful with these now-a-days substitute powders as many of there brands were never engraved on your firearm to use. Keep in mind substitutes always try to compare themselves to Black Powder in theory or measure. But in reality they don't quite make it too the same level as B/P for one reason or another. Many times than not it pays to be Old School about this stuff.