I would consider skipping part III and IV and just jump to VI to compute the full gain. Every year in the future compute the CA gain and give the client ONE 540ES to submit on April 15.

Go ahead and report the installment sale method for both Fed and CA when you do the return. Dependent upon the client income, it is possible it may cost them a bit more, but it is likely worth it to avoid the hassle. 

Keep it simple for us old folks (I just turned 76.)