If the buyer is not an individual and uses the application service within and without this state, calculated using any reasonable method, the sales are apportioned between the use in this state in proportion to the use of the application service in this state and the other states. If the location of a sale cannot be determined, the sale of an application service is in the state from which the order was placed in the regular course of the customer's business. If that office cannot be determined, the sales are considered received at the customer's billing address;
Neb. Rev. Stat. §§ 77-2734.14