So far I've just been playing ME3 single player. All the galaxy sections have been locked at 50%. Is that a global number or do the percentages rise as I play multiplayer? I hope it's the latter because, if it's the former, there clearly aren't enough people playing to make the numbers move at all.
If you join a multiplayer game where the map is set to random and your team survives all 10 waves, all sectors on your Galaxy map will gain an increase of 3-5% readiness. If the map is predefined before the round starts the the sector where said map is located will gain a 10% increase and the other sectors only a single percent increase.
After 10-15 successful games you should reach 100%. Please note that the rating constantly drops with 2-3 percent every day that you're not playing (no lower than the original 50%).