Since this value will change depending on your current HP stat, Survival Calc isn't as precise because the exact amount of HP percentage the Sub has will vary. For what it's worth, crunching the numbers myself, 12 HP / 52 Sp. Def will be the minimum EVs for 4 Sp. Atk Scald to not break, and 12 HP / 36 Sp. Def would be minimum 15/16 not to break.

exactly25% of the user's HP, but will round down, something like 0 HP Scizor at level 100, which hits 281 stat, will make a 70 HP Substitute, which is 70/281 = ~24.91%.