As scheduled the aircraft for your JFK flight comes off flight 1889 from ATL. I would check the status of that flight the morning you leave. Delta has 3 flights inbound from the US to STT. 2 come from ATL and 1 from JFK. Once all 3 flights are airborne from the US you can feel reasonably assured they will make the return from STT. I don’t think you will know that by 8:30 but you could check at that point and if they all show ontime departures scheduled at least have a good idea.