Charcoal is sold by weight. To compare briquettes to lump you have to use the same weight of each. An example is Alton Brown instructing his viewers to use x pound(s) of lump.
The problem most folks have with inconsistency is shape and density. Using a WSM I simply bought a second charcoal grate and turned it 90* to the original. When going for a long cook I use a fresh bag and dump it in. The big chunks are always on top therefore fill the bottom of the ring and the smaller chunks rest on the bigger chunks. I have had 16 + hour burns using low temps and a clay saucer in the water pan with coal leftover after snuffing the fire. Not using water causes me to close the vents more or even only using one vent therefore feeding the fire less oxygen so I burn less coal for the same temp.