Your calculation makes sense. However, for those 20 MB in flight, how much data is stored in buffers? I think that could be approximated by comparing ping time with speed of light.
e.g. if a 5000 km long link at 10 Gbit/s has latency of 0.0166 s, then you have zero buffering. If you have latency of 0.2 s, then around 2 Gbit of data must be stored in buffers somewhere, making 20 MB in flight only 1% of all data stored in such a ping filesystem.