The 2014 is "around" 21 kWh. The 2015 and 16 had a different battery supplier than the '14s, and are "around" 18.5 kWh.
65% of 21 is 13.65 (2014)
65% of 18.5 is 12.025 (2015)
60% of 18.5 is 11.1 (2016)
Well, I don't think it is quite as simple as you're saying. The 2014 battery was rated at 21.4 KWH with only 17.3 KWH usable which comes from here: https://www.gm-volt.com/attachments/slide5-jpg.164190/
That means they have intentionally crippled the battery to only reach 80.8% of its potential capacity. Some reviewers have computed the actual number to be 91% of potential capacity -- this is obviously a zealously guarded trade secret. If you take that into consideration, then 65% of "new" would be 11.245. On the other hand, GM could simply provide a "software update" to release the 19.2% of the battery which is crippled and would probably yield another 2 to 3 KWH of capacity to give you 2 to 3 more good years before you decide to junk the car. Chevrolet/GM might also provide preferential pricing to those who want to convert to the 2015/16 battery since I expect the street price of lithium ion battery packs will be even cheaper than it is now.
I am betting that a software update to provide us with extra range (i.e., the 19.2% they crippled given back to us) will be available in 2022.