To use this form effectively, report your total income and your business costs as of today. Report all of your deductions as you predict that they will be for the entire year. The result is what you would owe as of now. This can help you make quarterlies that are proportional to your income. If you'd like to be conservative you can specify deduction amounts that you've already paid in.
As you get more income, check back in and re-run to see how it changes. The site can save your information so you only have to make minor changes. No sign-on or e-mail required, just a cookie to remember you.