Va. Code Ann. § 58.1-412

The payroll factor is a fraction, the numerator of which is the total amount paid or accrued in the Commonwealth during the tax period by the corporation for compensation, and the denominator of which is the total compensation paid or accrued everywhere during the taxable year, to the extent that such payroll is used to produce Virginia taxable income and is effectively connected with the conduct of a trade or business within the United States and income therefrom is includable in federal taxable income.