First (and this is just my opinion - we all have one!) you really don't need to go through all that W-4 crap or bother with sending the IRS a 1040-ES via mail. You can pay your quarterly taxes to the IRS online any time you wish at www.irs.gov/payments. Just make sure you *PRINT* *YOUR* *RECEIPT* when done. On the website, just indicate you're making a 1040-ES quarterly payment and ensure you select the right tax year you're making the payment for.
In the TurboTax program you can manually enter the tax payments you made during the tax year you are filing for, under the Deductions & Credits tab in the "Estimates & Other Taxes Paid" section. So this isn't a big deal really.
Generally, most folks will fall into the 22% tax bracket with a small number in the 24% bracket. So if you send the IRS at least 20% of your "take" each quarter, then come tax filing time you'll be fine.
Now you can use the IRS worksheets or the TTX program if you want. But you'll always come out to between 19% and 23% of your as-of-yet untaxed income, being what you'll send to the IRS. So I myself don't waste my time and effort to "do the math", knowing that 20% minimum will suffice just fine. (It has for 15 years so far)
Finally, if your state also taxes personal income, you'll need to pay your state taxes quarterly too. Not all states offer the online payment option. So you'll have to check the webste for your state's taxation authority to see what your payment options are.
