Wash. Admin. Code § 458-28-040
Financial institutions having more than one place of business shall divide total taxable gross income so as to attribute taxable income to each location in the ratio of total interest earned (whether taxable or not) on loans originated at each location during the period covered by the tax return. The location at which a loan is originated is the place of business of the financial institution at which the customer deals with the financial institution to obtain the loan. Financial institutions having time or demand deposits may compute the ratio of total deposits at each location as a basis for approximating gross income of each location, provided the financial institution can demonstrate that the taxable income so computed will not differ by more than $10,000 in any one calendar year as to any one business location from the amount computed using the ratio of interest earned on loans originated at each location.
Wash. Admin. Code § 458-28-040
Order ET 72-1, § 458-28-040, filed 9/29/72.