First off Volume and weight are not the same for black powder subs; if your measure does not come close with black then maybe you should do some looking at other measures,I know they can vary a bit and be ok but I have one that is so far off I just use it to show people how bad some brands are.
As for how much powder you can burn in a barrel that changes with both the weight and the tightness of the sabot or bullet. I will say that according to the chrony shock waves from 200gr up show enough increase in velocity that I feel that they are reasonably efficant it is normal for their to be a peak in the burning curve of any powder and for it to taper dow from there with a 250 gr bullet I find the peak to be at about 125 gr.
Also remember that the important thing is accuracy, it really does not make any differance how powerfull a miss is. Lee