Well, technically the muscle in the penis when in the resting state is contracted, and it 'expands' to 'full size' when erect. If 0% is fully resting, then 100% would be fully 'awake'. This is slightly simplified, but you can consider a penis kind of like a sponge that remains 'slightly damp'. When enough bloodflow allows it to be 'wet', it expands to full size. When certain other conditions occur bloodflow can constrict and shrink it even more than normal and be 'dry'.