Yes, file the extension tonight. This will prevent a failure to file penalty.
If you expect to owe money (one of the questions on the 4868 extension), go to IRS Payments and make the payment. This can eliminate interest on the amount owed.
On the extension, you will be asked about your taxable income and how much you will owe. Approximate numbers are OK - the IRS is not going to go back and check when you actually file your return.
Make sense?
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