You can still file an extension and if you think you will some amount, pay that or pay nothing if you really don't know. What will happen is that you will not be charged any late filing fees for not filing by tonight's deadline, and any amount that you end up owing for 2015 will be assessed interest and fees for late payment. If you pay something tonight and end up getting a refund, the amount you pay now will be considered as payment and your refund will be calculated with that in mind.
