Hi @trymjb now you see why, in 1st reply to your original post after you said
“(I am writing about it in my masters thesis 
)”
I suggested
And gave you some big clues
“from the LoRaWAN specs and 1st principles” Note Not just the LoRa/Device specs
“spoilers: LoRaWAN Headers, Payload format and structures, Forward error correct/CRC check/Coding scheme etc. yields a SF specific staircase/quantised increase in total payload/on airtime”
Good luck with the rest of your thesis 
