If I followed you correctly, that percentage isn't for the overall campaign, but unique for each mission map.
It just measures how close you are to achieving the win condition. It starts above zero because you haven't won or lost. If it hits 0%, that means you've lost. 100% means you win. The exact mechanism used to calculate the percentage varies a bit, but most of the time it's based off how many planets the enemy has versus how many the enemy doesn't have. That's why it can change quite a bit without you doing anything- while you might be sitting tight, the other races are out there slugging it out.
With the mission you mentioned, Exploitation, I find it to be quite easy. The win condition is the removal of all Drengin and Yor planets, and your allies will do most of the work for you. I usually sit tight in my corner and turn into an economic/research juggernaut, paving the way for the others to wipe out the Drengin and Yor. I occasionaly won't even fire a shot, just sitting tight as the victory percentage creeps upward as the Drengin and Yor planets are conquered by the other races.
