Please ignore the Refund-O-Meter as it will probably confuse you ... instead refer to the actual form 1040 itself. Check it before and after making changes to see how those changes affects the overall return.
There are many things that you may add that may not change the bottom line at all ... some being adding more itemized deductions or credits. When your federal income tax is already zero then adding more expenses or non refundable credits will not change the bottom line at all. Many things cannot be changed ... SE taxes are one item that will not be affected by non refundable credits or itemized deductions so that "box" will not change no matter how much you wish it would.
