Best I can figure with pilot math, they withhold about 35% of the bonus to protect the innocent. I took my normal cumulative withholding percentage for taxes (~16% rounded) from my guarantee pay then subtracted that amount from total deductions. I then subtracted uniforms from total deductions. I then divided this resulatant number by the signing bonus amount to figure ~36% rounded withheld from the bonus.

I'm working with the assumption that per diem is not taxed (fairly certain it is not).

I don't know if this is how they actually figure the withholding on the signing bonus. It just seems like 36% is withheld after doing my brand of figuring.