Go to the "Continue" screen and pause the emulator.
Find your initial seed displayed in the lua script.
Open PokeFinder > Gen 3 Static > Generator tab.
Input the seed into the "Seed" box.
Set your target filters (shiny, IVs, nature, etc.) and click "Generate."
Method should be "Method 1."
If no results, adjust filters or reset for a new seed.
Choose a target advance from the results.
Step 2: Find the Delay
If you already know your Pokémon's delay (see Vlad's doc above), check the delay box in PokeFinder, enter it, generate again, and skip to Step 3. Otherwise, calculate it below.
Your player should be in front of the right legendary, NPC, or Pokéball.
Make a save state to avoid mistakes.
Advance to the final screen before the Pokémon is generated.
If there's a cry or dialog, that's usually the final screen.
At your target advance, press A to start the encounter or receive the Pokémon.
Check the IVs of the Pokémon.
In PokeFinder, input the IVs and find the advance you hit.
Calculate the delay: Delay = Target Advance - Advance Hit
Enter the delay in PokeFinder and generate again.
Note the new advance number.
Step 3: Get the Desired Pokémon
Retry using the new advance adjusted for the delay.
If successful, you'll hit the correct spread.
If not, check how many advances you were off, reload, adjust, and try again.