You have to figure out your total income and go from there. I don't recall filling out a W4--Instead, I entered a flat amount I wanted to have deducted from my check. Assuming your income will be solely from your pension, 10% of your monthly check would be a safe amount. It is easy to find a tax table online. The first $17,850 is taxed at 10%, and the next bracket up to $72,500 is taxed at 15%. However, the actual tax is lower because of exemptions and deductions. If you use Turbo Tax, just do a sample tax return with your estimated income, and don't forget your state tax.
The tricky part is if you plan to work part time after retirement. More than likely, you will need to enter an additional amount withheld from your check, even if you claim 0 Single. Unless you love to work, or need the money, the goal would be to stay in the 15% bracket. If there is just two of you, and you use the standard deduction, that would be around $90,000.