The computations of SSS, PhilHealth and HDMF are all based on monthly tables (or regulations). When deducting SSS, PhilHealth and HDMF on a semi-monthly basis, you just have to make sure that the total of the 1st and 2nd halves amount to what is required by the monthly tables.

The forum contains samples on how you can do this.

Many payroll officers base the computation of SSS, PhilHealth and HDMF on the monthly rate (not actual pay) of the employee because this is easier to do since the employee's rate is a fixed amount.

Other payroll officers that use payroll software compute it in a way that the deductions for the 1st half are based on the actual semi-monthly pay (not the rate) used with the monthly table. On the 2nd half, only the difference between what is needed by the whole month minus what has been deducted on the 1st half is applied.