For what it's worth, here is a bunch of data from a Uberti Walker. The measure is one of those adjustable volume things that seem to be calibrated for Goex. Swiss powder thrown from the same measure weights a few grains more due to density but when weighed, Swiss is still "hotter" than goex.
As to calibration, all I know is that I have used two chronographs on the same day on more than one occasion and the readings were very close together.
I note no fall off in accuracy between 55 and 60 grain charges or from one type of powder to the next but attempting to use very light charges could result in wild velocity variations because projectiles really need to be set down on top the powder column with possibly a bit of compression to get consitent ignition and burn. I have tried various combinations with and without overpowder wads finding no significant difference in velocities or round to round consistency. The better treated wads will keep the barrel clean for extended firing while leaving them off is ok too if you clean out the (considerable) powder fouling after shooting a chamber full. A cursory pass or two through the bore with a spit patch will do the trick.
Walker-Uberti
Charge Velocity Extreme Spread
{5 rounds}
140 Grain Ball
55 Gr/Vol Goex FFFg 1001 fps 54
60 Gr/Vol Goex FFFg 1115 fps 46
60 Gr/Vol Pyrodex P 1221 44
60 Gr/Vol A Pioneer 974 80
60 Gr/Vol Swiss FFFg 1278 53
55 Gr/Vol Swiss FFg 956 46
60 Gr/Vol PyrodexRS 1045 35
37 Gr/Vol H 777 1201 72
200 Grain Lee Bullet
40 Gr/Vol Goex FFFg 927 40
45 Gr/Vol Swiss FFFg 1074 33
45 Gr/Vol Pyrodex P 1087 47
170Grain Original Pattern Picket Bullet
40 Gr/Vol Swiss FFFg 1031 36
Energy 402 ft/lbs
45 Gr/Vol Goex FFFg 1026 84
45 Gr/Vol Swiss FFFg 1158 35
Energy 506 ft/lbs
45 Gr/Vol. Pyrodex P 1035 57
