The correct practice is to prorate CPF during the month when employee just received Permanent Resident (PR) status and it is based on the CPF rate for PR year 1 (PR1).
To determine the year of Singapore Permanent Resident (SPR) status for your employee, you can refer to this link here.
For the relevant CPF rates of a first and second year SPR, you can refer to this link here.
Proration process for a new SPR is automated in Talenox, here's how what you can do if this employee obtains SPR status mid-way through the month:
1) Head to Profiles > Manage my database > Employee Listing > Edit (New SPR's employee profile) > Identification Documents > Immigration Status.
2) Edit the new SPR's status to reflect "Singapore PR" and key in the PR Effective Date.
3) When you process payroll for the month for this new SPR, there'll be 2 rows of Basic Salary due to the automated salary proration feature based on the employee's job details.