Yup, not much point in getting too carried away in creating a VFO for the Bitx40.
Initial accuracy of the si5351bx routines can be calibrated to be quite good by adjusting si5351bx_vcoa
But a slight change in temperature and the 25mhz reference will drift a few hz,
so stability will be an issue

If an application comes up where greater stability is required,
the 25mhz (or 27mhz) reference could come from an ocxo or tcxo, or be GPS disciplined,
or perhaps you constantly adjust a knob for zero-beat with a known accurate radio station such as wwv.

Jerry, KE7ER