Two NL Payroll customers ask in the same week if one can activate the NL daily taxation tables. Coincidence.
While i answered the first time NO, except when an employee is hired or fired in the middle of the month, the second time i dived a little deeper in the seeded fast formulas and plsql functions, to discover a hidden fast formula to configure.
Define the fast formula NL_OVERRIDE_SOCIAL_INSURANCE_DAYS with the business logic required, and you have full control over the proration (daily tax) happening.