I want to gross-up bonus using the proper tax rates. I see the flat rate of 25%, but is that only federal income tax? Do I still need to include FICA and state rates?

I am trying to gross up a bonus using the proper tax rates. I used the TurboTax bonus calculator using the percentage method. We want the employee to net a $500 bonus. The calculator taxed the bonus at a flat rate of 25%, making the gross amount $625. However, I am receiving mixed information because other sites are stating that in addition to this flat 25% federal tax, I must include state bonus tax rates (NYS 9.62%) and FICA taxes. Is this correct or is it as simple as a 25% flat rate for federal income tax?