Since you asked.  I'll use Utah as an example, because they have a flat 5% state tax.  If a person in Utah has a taxable income of $100,000, and assuming the federal rate averages 15%, that person would pay $15,000 in federal income taxes and $5,000 (5% of $100,000) in state taxes.  The same person in Alaska would pay $2,250 in state taxes (15% of $15,000).  That's the difference between a tax based on federal taxable income and a tax based on federal taxes.  The proposed tax for Alaska is actually low, which makes sense if they want to get it passed.