You can use Pokegen to calculate a valid PID for your pokemon IV, Shiny, Nature and gender data. Probably a valid PID for the exact combination wont be possible, not every PID-IV combination is possible in gen3/4. Yo do not need to load the saves or the pokemon in Pokegen, Pokegen have a PID-IV calculator that will find a valid PID after selecting your wanted IV, Nature and other data, the PID can be copy from Pokegen and paste into PkHex. I recommend you to calculate a valid PID for your I