Line 1 of your Illinois IL-1040 reports your federal adjusted gross income. This is also Line 11 of your federal tax return. You may reference either of those to find the correct number to use for filing your Illinois tax return.
The language is a bit unclear as Illinois also has a "base income", which is Line 9 of the Illinois return - but the correct number to use is the federal adjusted gross income, found using either method listed above.
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