I was curious.......
Stage -1 = 0.67% Stage -2 = 0.50% Stage -3 = 0.40% Stage -4 = 0.33% Stage -5 = 0.29% Stage -6 = 0.25%
For example, if a Pokemon had an Attack stat of 320, and you lowered it to stage -2, that would be halving the stat, meaning it would have 160 Attack until the stage is brought back up.
http://bulbapedia.bulbagarden.net/wiki/Stats#Stages
One stage: .67x Two stages: .5x
So lets say you have a Pokemon with 312 attack s . At minus one, it would be the equivalent of a 208 attack, and at minus two, it would be lithe a 156 attack.
Hope this helps