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Substitute health always rounds down to a whole number. Thus, Entei's substitute has 100 HP, and Seismic Toss will do 100 damage. Entei would need at least 404 HP for its sub to live ST.
won ggs
https://replay.pokemonshowdown.com/gen9bssregf-2107516046-tsxd0ep218a3wep1mn9yb9sjlbb93obpw
https://replay.pokemonshowdown.com/gen9bssregf-2107516761-y9o47ssjm4nbgmbxmn3fcqdhuml5fr9pw
https://replay.pokemonshowdown.com/gen9bssregf-2107517167-bp69vdzuj9u9ahujamoo2gphtvlpea5pw
closing PS for the night; if you're available on fri/sat/sun during the windows i gave you can name a time then, otherwise i'm going to take an act win