TAX FORECAST: Is it legal?

Before the end of June 2011.Our company came up with "TAX FORECAST" which consists of my projected gross monthly income, tax exemptions, and other incentives from Jan to December 2011. After conducting computations, they arrived at a variance of 3400 for the tax I have to pay the whole year. in that case the manager told me that starting July until September they will raise my tax rate from 12% to 14% and I can't do anything about it. Either 14% or higher. In May, they decided to deduct 12% of my basic salary to be paid as my tax. Now they want me to pay 14%.
I told the manager that I'll just wait until december, she said no. It will take effect in July. Is there really a tax foreacast and is it valid? If not can I sue my employer if she insists on doing that? What complaint can I file against her. Honestly, I feel like being harassed since she wants me to pay my taxes for the months I haven't even worked for.

as long as the computation is correct, it is valid. the employer is obligated by law to deduct your income tax. this is done by computing your probable tax for the entire year and then dividing by 12.