I personally never judge how much powder and barrel X length can handle, or burn efficiently. I use what I call diminished returns. Lets say with bullet X I shoot 80, 90, even 100 grains, in barrel X, and after 100 grains the accuracy drops off. Well even if the barrel is four feet long, I know for that bullet, accuracy wise, after 100 grains, I get diminished returns. Meaning, no more accuracy improvement. So even if the rifle COULD burn more powder efficiently, there is no need to. So instead of trying to set a magic number on a length of barrel.. instead I see why the barrel looses accuracy with X bullet after a certain amount of powder.

Now for some range observations.. when I shot my 24 inch Knight barrel over snow, anything over 120 grains of powder gave me unburnt powder laying on top of the snow. And the diminished returns of that rifle was 110 grains of loose powder. So there was no problem.