This is the first time the pads are going to get changed. I've read that people generally change the front pads at least twice before changing the rears. I still have about 30-40% left in the front... haven't checked the rear yet... but I figure it should still have a good amount of "brakes" left.
Should I change both or just the fronts is OK?

 

i would suggest you to change both pads together, by changing the front, balance of the brake will be out, and if you only change the front pads, you will have even more oversteer when you do mid corner brake.