The primary difference between the California and federal computation methods for net income in Form 100 lies in the treatment of certain deductions and adjustments. While both methods start with the taxpayer's federal adjusted gross income (AGI), California allows for additional deductions and adjustments not permitted under federal tax laws. For example, California allows a deduction for certain business expenses paid with a credit card, while the federal tax code does not. Additionally, California may have different tax rates and brackets, which can impact the final net income and tax liability.