The thing is you first need to hit your seed so you can arrive at your MTIVRNG (the one for ivs), then you capture the first pokémon available in a place where preferably where there's no moving NPCs (I like the top of Celestial Tower for litwick). *Switch your synchronizer before attemting to capture, otherwise the captured could potentially have that nature. Then, check the ivs to confirm you hit your seed. Now, compare the captured pokémon with the PIDRNG around frames 40-60, there should be a perfect match, I usually guide myself by the ability and esv. Now, once you know where you're standing and know your calibration, restart game and get your seed again and chatot flip as many times as needed (the difference between shiny frame and calibrated frame) and you should find your shiny mon then. Of course, this is theoretically.