(1) ascertaining the percentage which the average value of the taxpayer's real and tangible personal property in its unrelated trade or business within the state during the period covered by the taxpayer's return bears to the average value of all the taxpayer's real and tangible personal property wherever situated during such period which is used in its unrelated trade or business (For purposes of this paragraph, the taxpayer's real property shall include not only such property owned by the taxpayer but also such property rented to it.);

(2) ascertaining the percentage which the receipts of the taxpayer's unrelated trade or business, computed on the cash or accrual basis according to the method of accounting used in the computation of the taxpayer's unrelated business taxable income, arising during such period from

(A) sales of tangible personal property by the unrelated trade or business where shipments are made to points within this state,

(B) services performed within the state by the unrelated trade or business,

(C) rentals from property of the unrelated trade or business situated within the state, and

(D) all other receipts earned by the unrelated trade or business within the state, bear to the total amount of the receipts of the unrelated trade or business, similarly computed, arising during such period from all sales of its tangible personal property, services, rentals and all other transactions, whether within or without the state;

(3) ascertaining the percentage of the total wages, salaries and other personal service compensation, similarly computed, during such period of employees of the taxpayer's unrelated trade or business within the state, except general executive officers, to the total wages, salaries and other personal service compensation, similarly computed, during such period of all employees of the unrelated trade or business within and without the state, except general executive officers; and

(4) adding together the percentages so determined and dividing the result by the number of percentages; provided, however, that if the taxpayer does not have a regular place of business outside the state in which its unrelated trade or business is conducted, the business allocation percentage shall be one hundred percent.

(b) If it shall appear to the tax commission that the allocation percentage determined in subdivision (a) of this section does not properly reflect the activity, business or income of a taxpayer's unrelated trade or business within the state, the tax commission shall be authorized, in its discretion, to adjust it by (1) excluding one or more of the factors therein, (2) including one or more other factors, such as expenses, purchases, contract values (minus subcontract values), (3) excluding one or more assets in computing such allocation percentage, provided the income therefrom is also excluded in determining unrelated business taxable income or (4) any other similar or different method calculated to effect a fair and proper allocation of the income reasonably attributable to this state. The tax commission from time to time shall publish all rulings of general interest with respect to any application of the provisions of this subdivision.