I am using my iPad Pro about 2.5 years.
If %8 is lost from iPad Battery in 2.5 years ( March 2016) is this a sign of a bad battery?
According to Apple, you're not bad at all and very much in the healthy range.
Your battery is designed to retain up to 80% of its original capacity at 1000 complete charge cycles. The one-year warranty includes service coverage for a defective battery. If it is out of warranty, Apple offers a battery service for $99, plus $6.95 shipping, subject to local tax.