The simple answer, which is probably the answer that you accountant told you, is to base your withholding on the number of exemptions you can claim. If you are single and have no dependent children, then you should fill out your W-4 for single and 1 allowance. Another easy answer is to go to this web address http://www.irs.gov/pub/irs-pdf/fw4.pdf which is the web address for Form W-4 and use the personal allowances worksheet to calculate the number of allowances you should claim. If this web address doesn't work for some reason, then go to www.irs.gov and click on Form W-4. Also, the expert who answered your question originally should have checked the IRS' link to their online withholding calculator. The address was correct, but for some reason the IRS' withholding calculator is down. You might try it again in the future and see if the IRS fixes the calculator.
Publication 505 "Tax Withholding and Estimated Tax" gives detailed directions on how to estimate your taxes for 2012 and check your withholding for accuracy. You can download this publication at this web address http://www.irs.gov/pub/irs-pdf/p505.pdf . If this web address doesn't work, then go to www.irs.gov , look under current year forms and publications and look for the Publication 505 by number. It has the current year estimated tax tables and gives step-by-step instructions on calculating your estimated tax and needed withholding in Chapter 1.
If you determine that you need to change your withholding, remember that you are already in the third quarter of the year, so you have November and December to make up any underwithholding for the entire year. You will need to file a new W-4 form with your employer either electronically or on paper.
Under the IRS rules, if you have withholding within $1000 of your taxes due, or if you have withheld 90% of the taxes due, or if you have withheld 100% of the total taxes due last year if you make less than $150,000 or 110% if you make more than $150,000 (note that this is total tax due not the amount of refund or balance due from last year), then while you will have to pay the balance due at tax time, you won't be penalized for underwithholding.
I hope this answers your questions. Please let me know if you need more information about this question or if you need me to clarify anything for you.