North Carolina Administrative Code Title 17 — Revenue
17 NCAC 05g .0803 — Reasonable Approximation
When the taxpayer cannot determine the state or states where a service was received pursuant to Rule .0802 of this Section, but the taxpayer has information regarding the location of receipt from which the taxpayer can reasonably approximate the state or states where the service is received, the taxpayer shall reasonably approximate such state or states. section .0900 - services delivered to a customer or on behalf of the customer, or delivered electronically through the customer
Source: official text