How California's tax environment affects the W2 vs 1099 comparison.
California has a state income tax rate up to 13.3% at higher incomes, one of the highest in the US. Both W2 and 1099 workers pay the same state income tax rate on taxable income — the difference is entirely in federal SE tax and how deductions work. CA also has SDI (State Disability Insurance) that W2 employees pay (currently ~0.9% of wages).
Use 9.3% as your state income tax rate in the calculator.