How to calculate SSBFREQPOS number in 5G NR?

Hi Experts…
Why the SSBFREQPOS is different from DL/ULNARFCN in 5G?
How can I calculate the SSBFREQPOS number?

SSB position number

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Extra (additional) reading about GSCN:

http://www.sharetechnote.com/html/5G/5G_FR_Bandwidth.html#GSCN