This is also how I calculate it. By the way, I have a Mix 2 and when my phone is below 20% I charge it and it takes about 1 to 1.5 hours. So the 2 hours from absolute zero sounds about right. Only when people ask for support for fast charge they probably mean other charge-adapters with higher voltage/ampere output. I received a 5V/2A charger and for me that is fine. But other phones can charge to 80% in about half an hour because of different/better charge-adapters. So what a lot of people want to know is whether a phone like the Mix 2 supports other charge-adapters which are faster.