On cart we used 0 spa choice specs iron bundle and 0hp/0spdef Miraidon, on PS we used an Azumarill with the same stats as the ingame Miraidon (0/0 evs with unaffecting nature) to emulate the exact same situation (lvl 50 175 hp 135 SpD + resists water). The way they're currently stacking is by first subtracting 25% of the mons' spdef, and then subtracting another 25% from that number.
0 SpA Choice Specs Iron Bundle Hydro Pump vs. 0 HP / 0 SpD Ruin Miraidon: 66-78 (37.7 - 44.5%) -- guaranteed 3HKO (51.20% chance to 3HKO after accuracy) (66, 67, 68, 69, 69, 70, 71, 72, 72, 73, 74, 75, 75, 76, 77, 78)
0 SpA Choice Specs Iron Bundle Hydro Pump vs. 0 HP / 180+ SpD Ruin Azumarill: 66-78 (37.7 - 44.5%) -- guaranteed 3HKO (51.20% chance to 3HKO after accuracy) (66, 67, 68, 69, 69, 70, 71, 72, 72, 73, 74, 75, 75, 76, 77, 78)