Hi Tim,
indeed I did not take into account the impact on the payload because for LoRa it is no so straight forward since it depends on SF and coding rate.
For Sigfox a 12 byte payload results in a 25 byte message on air.
For . http://www.semtech.com/forum/images/datasheet/LoraDesignGuide_STD.pdf gives some information. on top of the PHY header there is the mac overhead of 5 bytes (header of 1 byte, a MIC of 4 byte) then you have to add the PHY headers, CRC (only in uplink) and preamble (0 to 8 bytes). Then then frame header. In total you need at least 13 extra byte but can be more a good discussion is on Spreadsheet for LoRa airtime calculation
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