The Filling Percentage is the effective or usable combustion camber volume occupied by the charge. This is also also named "Load Ratio" or "Fill Rate" and con be found in most reloading manuals.

The Effective Combustion Chamber Volume is further explained here.

The volume occupied by the charge is equal to the powder bulk density multiplied by the weight of the charge. The powder bulk density can be found in our powder/propellant database.

Important notice: Reloaders commonly try to operat with fill rates between 80 and 100%. Loads below 80% can be dangerous as they can produce very unsteady results. Loads above 100% are "press loads". They also produce unsteady results and often dangerous pressures.