GA S-Corp donig Business in California

When a foreign (Georgia) S-Corp conducts business in California, what service income should be used to apportion income to California?

Facts - the S-Corp was formed in GA but is registered to conduct business in CA. For the year in question, roughly 10% of the services were provided inside CA while 90% was outside CA. The sole shareholder/employee is a CA resident and performs all of the services. FWIW, the shareholder is not an officer. The officers reside in GA and manage a stock portfolio for the S-Corp as well as helping the shareholder during travels to other states/countries to provide services.

I think that only the services provided in CA should be used; however, I want to confirm this belief.

Thanks,

Both CA and GA are market based sourcing states (as opposed to cost of performance).

You should source revenue based on where the client/customer received the benefit of the service.