How SIL Rating of a SIF is decided? SIL- Safety Integrity Level

How SIL Rating of a SIF is decided? SIL- Safety Integrity Level
As per IEC-61511 Safety Life Cycle, PHA is done first. PHA/HAZOP gives qualitative risk. Then LOPA is conducted. LOPA identifies different protection layers and risk mitigated by each layer . Further derivation provides the Risk Reduction ( A) by all layer except Safety Instrumented Function There is a tolerable risk of each organization. In some cases , country specific regulations provide tolerable risk (B). So, you need to achieve B and you already achieved A. So, Risk Reduction Factor (RRF) for SIF becomes A/B.
Now if RRF is more than 10 and less than equal to 100, the Safety Integrity level of the SIF is 1 or SIL 1.Now if RRF is more than 100 and less than equal to 1000, the Safety Integrity level of the SIF is 2 or SIL 2.Now if RRF is more than 1000 and less than equal to 10000, the Safety Integrity level of the SIF is 3 or SIL 3.