How do you compute withholding tax?

There are many ways to do so...questions on why your withholding is what it is should be addressed to whoever is making the calculation, which is done pursuant to the info you provide on your W-4. AGAIN, IT IS CONTROLLED BY YOU...NOT SOME OUTSIDE or SET AMOUNT.

If you need to adjust it, because your circumstances require more or less to be withheld to approximate your liability after other income, or special dedcutions you may have...that can be done through them. Understand that many things are withheld from pay, some but not all are tax...and not all people call the same things "tax". Is FICA contributions tax? Many don't think so. Or unemployment contributions? Etc. So you have to define what it is you're speaking about. The amount wittheld, like the amount of tax actually determined to be due, is dependent on many, many factors...such as your maritial and family status, other income and expenses, how other employer beenfits are handled (like retirement contributions, health and medical, etc., etc). It is very reasonable to say that even 2 people at the same job making the same salary will commonly have very different amounts withheld.