It’s not ‘proprietary’ in ImReady4. It’s a reverse engineered calculation from what iThlete HRV is doing which I made in Excel, and that was later on implemented in ImReady4, which is a lot more user-friendly then the Excel version.
The Excel template is freely available here
The result is NOT a ‘Readiness’ score. It’s one out of 4 training advice’s, Rest-LIT-Planned-HIT.
The ‘formula’ is a rather complicated nested ‘If-Then-Else’ based on the Z-score of restingHR and HRV.