65535 tREFI is only recommended if you can keep the memory running relatively cool (under 50°C SPD temp during stress test); this value most likely requires active memory cooling.
A more stable value without active cooling is 40959
>5 refresh periods * 8192 clock cycles in a single refresh period - 1
The subtraction accounts for zero-based indexing of the internal refresh counter.
If tREFI was set to 8192, the refresh would occur at the 8193rd cycle.

The PC in the image is not mine.