As I am dependent on the FT817's internal TCXO, drift may well be an issue. As you can see, drift is a real problem. In the end I set the FT817 100Hz low which seemed where I was centred on the WSPR segment.
On WSJT-X V1.7 you can see about 2.5kHz of the band in real time, so I just made the frequency adjustment to where most activity seemed to be. This could be wrong and I could drift off!
Just exchanged WSPR spots with G8OFA (189km).