I had a similar issue in May. Mine would be around 80-82%. The tech said he can offer a warranty battery replacement even if his side showed him 83% or something. I told him that my Mac only lasts 3 hours and if thats normal? He said no and said he can make an exception. I say go take it in or call Apple support and politely tell them that the battery is sinking and you feel you were sold a faulty Mac. They can mail you a box and they have fast turn around service. Id do this in maybe mid Oct. let that % sink or you can even cal them now. they log the claims so even if they say 'no' at least when you call back closer to expiration, they can see that you had pervious complaints.
Personally I wouldnt get apple warranty on this Mac since the sale prices are dropping because of the new Macs coming out. Especially if you keep it in the house most of the time.