If you claim married and no withholding allowances your federal income tax withheld using the percentage method would be about $3,195 or 25.56% of your $12,500 semi-monthly pay. FICA withholding will amount to an additiona 7.65% until you reach the $92,400 social security limit. After that the FICA withholding percentage drops to 1.45%.
If you claim one withholding allowance your federal income tax withheld will decrease to $3,150 or 25.19%.
I completed some calculations using the information you gave me to give you some perspective. Assuming a $300,000 salary, married filing joint and claiming the standard deduction, your estimated federal income tax liability will be about $78,400. Adding a child gains you no tax benefit (i.e. from either the dependent exemption or child tax credit) because your income level is such that the exemption and credit is phased out. Further, your subject to AMT which makes your effetcive federal tax rate 28%.
Your best bet to reduce your tax burden is to shelter as much income as you can through all the options available through your employer. This includes maxing 401(k) contributions, invest excess cash in tax-exempts and annuities, defer salary through non-qualified plans, take bonuses in options or restricted stock (do Sec. 83(b) elections), etc.
Because it is impossible for me to identify and consider ALL the relevant facts, this advice is not intended or written to be used for the purpose of avoiding penalties, and cannot be used for that purpose.