The exemption given by the government is for the whole year. You can see the exemptions applied to the computation if you use the Yearly mode of the withholding tax table.

You can probably apply the exemptions to the semi-monthly table, too. But you should divide the exemption (for the year) by 24. (But the computations would still be estimates unlike using the annual exemption with annual pay)

Example:
Semi-monthly Exemption for ME3 = 125,000 / 24 =5208.33

Where:
125,000 is the Annual Exemption for ME3
24 is the number of semi-monthly periods for the year