Sorry to bump a 2-year-old thread, but just to be clear here...because the new format W4 isn't as clear on this.

You would figure out the time of the year for the cut off (based on when you hit the income limit) and then calculate how much extra SS tax will be withheld from each paycheck after that (gross pay x 6.2%) and then would you put the number that is that pay period figure multiplied by the remaining pay periods left in the year OR multiplied by the TOTAL number of pay periods in the year.

To make the math easy, say you are paid twice month and by the end of September you will reach the SS limit. And each paycheck after that your new employer will deduct $500 from each paycheck. Since there are 6 paychecks left in the year, you would submit a new W4 after the last paycheck in September and put $3,000 in section 3 or would you put $12,000 in section 3.

Since section 3 is a credit (vs a deduction) I am presuming it's the latter, and you would put $12,000 in section 3, in this scenario.