The Spreadsheet for LoRa airtime calculation (or a work-in-progress online version) and TTN’s calculator (use 15 for the input; it adds 13 bytes for the header) also show 1646.6 ms. That’s all I know. 
The Spreadsheet for LoRa airtime calculation (or a work-in-progress online version) and TTN’s calculator (use 15 for the input; it adds 13 bytes for the header) also show 1646.6 ms. That’s all I know.