How to reduce payload size

[cslorabox]

@descartes

But the air time boundaries aren’t uniform across the different data rates, so DR5 has two bytes before increasing, then has 4 bytes, whereas DR3’s window is the first four bytes.

That does not match my recollection of the airtime spreadsheet - do you have a cite for data rate being involved and not simply 4/5 coding granularity with the count of groups scaled by the data rate to determine airtime?