Recently I've been trying to adjust my W-4 so that I overpay the government less (get a smaller tax return). How can I calculate the number of my deductions based on my income and family status, charitable giving, etc to arrive at the minimal overpayment?
A W4 allowance is like saying "don't tax $3800 on me, bro." So, if you are in the 25% bracket, raising it by 1 will have you pay $950 less in a year. 15% bracket? The same extra allowance lowers tax withheld by $540.
An article I wrote goes into some depth on the process.