How do Unicast/Broadcast hop times vary with message length?

The XBee Pro SX manual provides this calculation for unicast transmission timeout with a known route:
knownRouteUnicastTime=2 * NH * MR * unicastOneHopTime

The unicastOneHopTime (%H) value I can query via XCTU, e.g. 151ms when BR=2 (250kbps). But I’m confused since I’d expect this value to vary with message length. Do the %H and %8 values reported assume a particular message length?

I did find this page listing total transmission times for messages of varying length, 1byte and 32bytes including for XBee SX devices. And I do observe the expected variation in transmission time by payload size.

But for more accurately computing values for %H and %8, is there a clear relation that could be used to scale them up to larger message lengths like 255bytes?

It is per RF packet. So as long as your data packet is sent to the radio’s UART as one packet and not being sent in chunks with RO character times of silence between them, they will be sent as one packet as long as it does not exceed the maximum payload.