Yes, it's an automatic extension to file your return. It's not an extension to pay tax, so those lines are there to help you through the process of figuring out what you should pay. There's only penalties and interest if you don't pay in by the 18th. If you only had W-2 income and usually don't owe money, you should be ok, but get it filed asap!
You can look at your pay stubs near the end of the year and they have a year-to-date total on them of your income and withholding. You can enter this info into TurboTax and see what your federal estimated tax comes out to.