Del. Code tit. 30, § 1121

A tax is hereby imposed for each taxable year on the taxable income of every nonresident individual of this State equal to the tax determined under § 1102 of this title as if such individual were a resident, reduced by the credit allowed under § 1110(b) of this title, and the difference, multiplied by a fraction, the numerator of which is such individual’s modified Delaware source income and the denominator of which is such individual’s Delaware adjusted gross income.