You have no idea of how far off the synchronization is.
It could be off by minutes. The crew pulled the breaker on the processor, and all that was running was a periodic handshake.
What synchronization would that be?
The only relevant delay is how long it takes the gadget on the airplane to turn around a ping. And, since Inmarsat probably supplies the box, they would have a good read on that. And, if not, Boeing would know where to get the information. After that, it's easy: Time the ping response, subtract out turn around delay, divide by two, and multiply by the speed of light.