Calculating fair-use parameters

[SoothingMist]

This may be an offshoot of https://github.com/sandeepmistry/arduino-LoRa/issues/482.

Am using this calculator: https://avbentem.github.io/airtime-calculator/ttn/us915/222. I can see how to set SF, BW, and Frequency. However, am not sure how to set overhead size. Is that the same as what this library calls "preamble"?

Each LoRaP2P message (packet) has an envelope and contents (payload). I assume that "overhead" is the same as "envelope".

For the purposes of this library, how does one calculate overhead?

Many thanks for the community's insights.