I didn't read this whole thread so it might have already been said- If you are based in CA but do not live there then more than 50% of you flying has to have been performed in (over) the sate of CA in order for them to tax you. If 50% or more was outside the the state of CA then they have no claim to your income. The reg is out there and it should come up on google or even the state tax franchise board site.

Copy the reg and send it along with your letter stating where the majority of flying was done.