35 Miss. Code R. § 3-08-06-401.03
ALLOCATION OF INCOME. Any taxpayer subject to the taxing jurisdiction of this state shall allocate non-business income or loss within and without this state in accordance with the further provisions of this Regulation. All expenses connected with earning non-business income, such as interest, taxes, general and administrative expenses and such other expenses relating to the production of non-business income, shall be deducted from gross non-business income. Non-business interest expense shall be computed by using the ratio of non-business assets to total assets applied to total interest expense. To the amount of non-business income allocated to this state, there shall be added the amount of net business income assigned, directly allocated or apportioned to this state under the other provisions of this Regulation to establish Mississippi taxable income.
35 Miss. Code. R. § 3-08-06-401.03